diff -u b/core/includes/image.inc b/core/includes/image.inc
--- b/core/includes/image.inc
+++ b/core/includes/image.inc
@@ -38,10 +38,10 @@
 /**
  * Gets a list of available toolkits.
  *
- * @return
+ * @return array
  *   An array with the toolkit names as keys and the descriptions as values.
  */
-function image_get_available_toolkits($key = NULL) {
+function image_get_available_toolkits() {
   // Use plugin system to get list of available toolkits.
   $toolkits = drupal_container()->get('image.toolkit.manager')->getDefinitions();
 
@@ -57,9 +57,13 @@
 }
 
 /**
- * Gets the name of the currently used toolkit.
+ * Gets an image toolkit.
  *
- * @return
+ * @param $toolkit_id
+ *   (optional) String specifying toolkit to load. NULL will load the default
+ *   toolkit.
+ *
+ * @return Drupal\system\Plugin\ImageToolkitInterface
  *   Object of the default toolkit, or FALSE on error.
  */
 function image_get_toolkit($toolkit_id = NULL) {
@@ -94,12 +98,12 @@
  * toolkit, and may support others, depending on which toolkits are
  * installed.
  *
- * @param $filepath
+ * @param string $filepath
  *   String specifying the path of the image file.
- * @param $toolkit
- *   An optional image toolkit object to override the default.
+ * @param Drupal\system\Plugin\ImageToolkitInterface $toolkit
+ *   (optional) An image toolkit object to override the default.
  *
- * @return
+ * @return stdClass
  *   FALSE, if the file could not be found or is not an image. Otherwise, a
  *   keyed array containing information about the image:
  *   - "width": Width, in pixels.
@@ -108,13 +112,13 @@
  *   - "mime_type": MIME type ('image/jpeg', 'image/gif', 'image/png').
  *   - "file_size": File size in bytes.
  */
-function image_get_info($filepath, $toolkit = FALSE) {
+function image_get_info($filepath, Drupal\system\Plugin\ImageToolkitInterface $toolkit = NULL) {
   $details = FALSE;
   if (!is_file($filepath) && !is_uploaded_file($filepath)) {
     return $details;
   }
 
-  if (!$toolkit) {
+  if ($toolkit === NULL) {
     $toolkit = image_get_toolkit();
   }
   if ($toolkit) {
@@ -139,14 +143,14 @@
  *
  * The resulting image always has the exact target dimensions.
  *
- * @param $image
+ * @param stdClass $image
  *   An image object returned by image_load().
- * @param $width
+ * @param int $width
  *   The target width, in pixels.
- * @param $height
+ * @param int $height
  *   The target height, in pixels.
  *
- * @return
+ * @return bool
  *   TRUE on success, FALSE on failure.
  *
  * @see image_load()
@@ -169,20 +173,21 @@
  *
  * The resulting dimensions can be smaller for one or both target dimensions.
  *
- * @param $dimensions
+ * @param array $dimensions
  *   Dimensions to be modified - an array with components width and height, in
  *   pixels.
- * @param $width
- *   The target width, in pixels. If this value is NULL then the scaling will be
- *   based only on the height value.
- * @param $height
- *   The target height, in pixels. If this value is NULL then the scaling will
- *   be based only on the width value.
- * @param $upscale
- *   Boolean indicating that images smaller than the target dimensions will be
- *   scaled up. This generally results in a low quality image.
+ * @param int $width
+ *   (optional) The target width, in pixels. If this value is NULL then the
+ *   scaling will be based only on the height value.
+ * @param int $height
+ *   (optional) The target height, in pixels. If this value is NULL then the
+ *   scaling will be based only on the width value.
+ * @param bool $upscale
+ *   (optional) Boolean indicating that images smaller than the target
+ *   dimensions will be scaled up. This generally results in a low quality
+ *   image.
  *
- * @return
+ * @return bool
  *   TRUE if $dimensions was modified, FALSE otherwise.
  *
  * @see image_scale()
@@ -217,19 +222,19 @@
  *
  * The resulting image can be smaller for one or both target dimensions.
  *
- * @param $image
+ * @param stdClass $image
  *   An image object returned by image_load().
- * @param $width
- *   The target width, in pixels. This value is omitted then the scaling will
- *   based only on the height value.
- * @param $height
- *   The target height, in pixels. This value is omitted then the scaling will
- *   based only on the width value.
- * @param $upscale
- *   Boolean indicating that files smaller than the dimensions will be scaled
- *   up. This generally results in a low quality image.
+ * @param int $width
+ *   (optional) The target width, in pixels. This value is omitted then the
+ *   scaling will based only on the height value.
+ * @param int $height
+ *   (optional) The target height, in pixels. This value is omitted then the
+ *   scaling will based only on the width value.
+ * @param bool $upscale
+ *   (optional) Boolean indicating that files smaller than the dimensions will
+ *   be scaled up. This generally results in a low quality image.
  *
- * @return
+ * @return bool
  *   TRUE on success, FALSE on failure.
  *
  * @see image_dimensions_scale()
@@ -250,14 +255,14 @@
 /**
  * Resizes an image to the given dimensions (ignoring aspect ratio).
  *
- * @param $image
+ * @param stdClass $image
  *   An image object returned by image_load().
- * @param $width
+ * @param int $width
  *   The target width, in pixels.
- * @param $height
+ * @param int $height
  *   The target height, in pixels.
  *
- * @return
+ * @return bool
  *   TRUE on success, FALSE on failure.
  *
  * @see image_load()
@@ -273,18 +278,18 @@
 /**
  * Rotates an image by the given number of degrees.
  *
- * @param $image
+ * @param stdClass $image
  *   An image object returned by image_load().
- * @param $degrees
+ * @param int $degrees
  *   The number of (clockwise) degrees to rotate the image.
- * @param $background
- *   An hexadecimal integer specifying the background color to use for the
- *   uncovered area of the image after the rotation. E.g. 0x000000 for black,
- *   0xff00ff for magenta, and 0xffffff for white. For images that support
- *   transparency, this will default to transparent. Otherwise it will
+ * @param string $background
+ *   (optional) An hexadecimal integer specifying the background color to use
+ *   for the uncovered area of the image after the rotation. E.g. 0x000000 for
+ *   black, 0xff00ff for magenta, and 0xffffff for white. For images that
+ *   support transparency, this will default to transparent. Otherwise it will
  *   be white.
  *
- * @return
+ * @return bool
  *   TRUE on success, FALSE on failure.
  *
  * @see image_load()
@@ -297,18 +302,18 @@
 /**
  * Crops an image to a rectangle specified by the given dimensions.
  *
- * @param $image
+ * @param stdClass $image
  *   An image object returned by image_load().
- * @param $x
+ * @param int $x
  *   The top left coordinate, in pixels, of the crop area (x axis value).
- * @param $y
+ * @param int $y
  *   The top left coordinate, in pixels, of the crop area (y axis value).
- * @param $width
+ * @param int $width
  *   The target width, in pixels.
- * @param $height
+ * @param int $height
  *   The target height, in pixels.
  *
- * @return
+ * @return bool
  *   TRUE on success, FALSE on failure.
  *
  * @see image_load()
@@ -329,10 +334,10 @@
 /**
  * Converts an image to grayscale.
  *
- * @param $image
+ * @param stdClass $image
  *   An image object returned by image_load().
  *
- * @return
+ * @return bool
  *   TRUE on success, FALSE on failure.
  *
  * @see image_load()
@@ -347,12 +352,12 @@
  *
  * Any changes to the file are not saved until image_save() is called.
  *
- * @param $file
+ * @param string $file
  *   Path to an image file.
- * @param $toolkit
- *   An optional, image toolkit object to override the default.
+ * @param Drupal\system\Plugin\ImageToolkitInterface $toolkit
+ *   (optional) Image toolkit object to override the default.
  *
- * @return
+ * @return stdClass
  *   An image object or FALSE if there was a problem loading the file. The
  *   image object has the following properties:
  *    - 'source' - The original file path.
@@ -367,8 +372,8 @@
  * @see image_get_available_toolkits()
  * @see image_gd_load()
  */
-function image_load($file, $toolkit = FALSE) {
-  if (!$toolkit) {
+function image_load($file, Drupal\system\Plugin\ImageToolkitInterface $toolkit = NULL) {
+  if ($toolkit === NULL) {
     $toolkit = image_get_toolkit();
   }
   if ($toolkit) {
@@ -388,14 +393,14 @@
 /**
  * Closes the image and saves the changes to a file.
  *
- * @param $image
+ * @param stdClass $image
  *   An image object returned by image_load(). The object's 'info' property
  *   will be updated if the file is saved successfully.
  * @param $destination
- *   Destination path where the image should be saved. If it is empty the
- *   original image file will be overwritten.
+ *   (optional) Destination path where the image should be saved. If it is empty
+ *   the original image file will be overwritten.
  *
- * @return
+ * @return bool
  *   TRUE on success, FALSE on failure.
  *
  * @see image_load()
diff -u b/core/modules/image/image.effects.inc b/core/modules/image/image.effects.inc
--- b/core/modules/image/image.effects.inc
+++ b/core/modules/image/image.effects.inc
@@ -64,20 +64,20 @@
 /**
  * Image effect callback; Resize an image resource.
  *
- * @param $image
+ * @param stdClass $image
  *   An image object returned by image_load().
- * @param $data
+ * @param array $data
  *   An array of attributes to use when performing the resize effect with the
  *   following items:
  *   - "width": An integer representing the desired width in pixels.
  *   - "height": An integer representing the desired height in pixels.
  *
- * @return
+ * @return bool
  *   TRUE on success. FALSE on failure to resize image.
  *
  * @see image_resize()
  */
-function image_resize_effect(&$image, $data) {
+function image_resize_effect(stdClass &$image, array $data) {
   if (!image_resize($image, $data['width'], $data['height'])) {
     watchdog('image', 'Image resize failed using the %toolkit toolkit on %path (%mimetype, %dimensions)', array('%toolkit' => $image->toolkit->getPluginId(), '%path' => $image->source, '%mimetype' => $image->info['mime_type'], '%dimensions' => $image->info['width'] . 'x' . $image->info['height']), WATCHDOG_ERROR);
     return FALSE;
@@ -88,10 +88,10 @@
 /**
  * Image dimensions callback; Resize.
  *
- * @param $dimensions
+ * @param array $dimensions
  *   Dimensions to be modified - an array with components width and height, in
  *   pixels.
- * @param $data
+ * @param array $data
  *   An array of attributes to use when performing the resize effect with the
  *   following items:
  *   - "width": An integer representing the desired width in pixels.
@@ -106,9 +106,9 @@
 /**
  * Image effect callback; Scale an image resource.
  *
- * @param $image
+ * @param stdClass $image
  *   An image object returned by image_load().
- * @param $data
+ * @param array $data
  *   An array of attributes to use when performing the scale effect with the
  *   following items:
  *   - "width": An integer representing the desired width in pixels.
@@ -116,12 +116,12 @@
  *   - "upscale": A boolean indicating that the image should be upscaled if the
  *     dimensions are larger than the original image.
  *
- * @return
+ * @return bool
  *   TRUE on success. FALSE on failure to scale image.
  *
  * @see image_scale()
  */
-function image_scale_effect(&$image, $data) {
+function image_scale_effect(stdClass &$image, array $data) {
   // Set sane default values.
   $data += array(
     'width' => NULL,
@@ -139,10 +139,10 @@
 /**
  * Image dimensions callback; Scale.
  *
- * @param $dimensions
+ * @param array $dimensions
  *   Dimensions to be modified - an array with components width and height, in
  *   pixels.
- * @param $data
+ * @param array $data
  *   An array of attributes to use when performing the scale effect with the
  *   following items:
  *   - "width": An integer representing the desired width in pixels.
@@ -159,9 +159,9 @@
 /**
  * Image effect callback; Crop an image resource.
  *
- * @param $image
+ * @param stdClass $image
  *   An image object returned by image_load().
- * @param $data
+ * @param array $data
  *   An array of attributes to use when performing the crop effect with the
  *   following items:
  *   - "width": An integer representing the desired width in pixels.
@@ -170,11 +170,13 @@
  *     of "XOFFSET-YOFFSET". XOFFSET is either a number of pixels or
  *     "left", "center", "right" and YOFFSET is either a number of pixels or
  *     "top", "center", "bottom".
- * @return
+ *
+ * @return bool
  *   TRUE on success. FALSE on failure to crop image.
+ *
  * @see image_crop()
  */
-function image_crop_effect(&$image, $data) {
+function image_crop_effect(stdClass &$image, array $data) {
   // Set sane default values.
   $data += array(
     'anchor' => 'center-center',
@@ -193,18 +195,20 @@
 /**
  * Image effect callback; Scale and crop an image resource.
  *
- * @param $image
+ * @param stdClass $image
  *   An image object returned by image_load().
- * @param $data
+ * @param array $data
  *   An array of attributes to use when performing the scale and crop effect
  *   with the following items:
  *   - "width": An integer representing the desired width in pixels.
  *   - "height": An integer representing the desired height in pixels.
- * @return
+ *
+ * @return bool
  *   TRUE on success. FALSE on failure to scale and crop image.
+ *
  * @see image_scale_and_crop()
  */
-function image_scale_and_crop_effect(&$image, $data) {
+function image_scale_and_crop_effect(stdClass &$image, array $data) {
   if (!image_scale_and_crop($image, $data['width'], $data['height'])) {
     watchdog('image', 'Image scale and crop failed using the %toolkit toolkit on %path (%mimetype, %dimensions)', array('%toolkit' => $image->toolkit->getPluginId(), '%path' => $image->source, '%mimetype' => $image->info['mime_type'], '%dimensions' => $image->info['width'] . 'x' . $image->info['height']), WATCHDOG_ERROR);
     return FALSE;
@@ -215,15 +219,17 @@
 /**
  * Image effect callback; Desaturate (grayscale) an image resource.
  *
- * @param $image
+ * @param stdClass $image
  *   An image object returned by image_load().
- * @param $data
+ * @param array $data
  *   An array of attributes to use when performing the desaturate effect.
- * @return
+ *
+ * @return bool
  *   TRUE on success. FALSE on failure to desaturate image.
+ *
  * @see image_desaturate()
  */
-function image_desaturate_effect(&$image, $data) {
+function image_desaturate_effect(stdClass &$image, array $data) {
   if (!image_desaturate($image)) {
     watchdog('image', 'Image desaturate failed using the %toolkit toolkit on %path (%mimetype, %dimensions)', array('%toolkit' => $image->toolkit->getPluginId(), '%path' => $image->source, '%mimetype' => $image->info['mime_type'], '%dimensions' => $image->info['width'] . 'x' . $image->info['height']), WATCHDOG_ERROR);
     return FALSE;
@@ -234,9 +240,9 @@
 /**
  * Image effect callback; Rotate an image resource.
  *
- * @param $image
+ * @param stdClass $image
  *   An image object returned by image_load().
- * @param $data
+ * @param array $data
  *   An array of attributes to use when performing the rotate effect containing
  *   the following items:
  *   - "degrees": The number of (clockwise) degrees to rotate the image.
@@ -246,11 +252,13 @@
  *   - "bgcolor": The background color to use for exposed areas of the image.
  *     Use web-style hex colors (#FFFFFF for white, #000000 for black). Leave
  *     blank for transparency on image types that support it.
- * @return
+ *
+ * @return bool
  *   TRUE on success. FALSE on failure to rotate image.
+ *
  * @see image_rotate().
  */
-function image_rotate_effect(&$image, $data) {
+function image_rotate_effect(stdClass &$image, array $data) {
   // Set sane default values.
   $data += array(
     'degrees' => 0,
@@ -287,10 +295,10 @@
 /**
  * Image dimensions callback; Rotate.
  *
- * @param $dimensions
+ * @param array $dimensions
  *   Dimensions to be modified - an array with components width and height, in
  *   pixels.
- * @param $data
+ * @param array $data
  *   An array of attributes to use when performing the rotate effect containing
  *   the following items:
  *   - "degrees": The number of (clockwise) degrees to rotate the image.
diff -u b/core/modules/system/lib/Drupal/system/Plugin/ImageToolkitInterface.php b/core/modules/system/lib/Drupal/system/Plugin/ImageToolkitInterface.php
--- b/core/modules/system/lib/Drupal/system/Plugin/ImageToolkitInterface.php
+++ b/core/modules/system/lib/Drupal/system/Plugin/ImageToolkitInterface.php
@@ -34,14 +34,15 @@
   /**
    * Scales an image to the specified size.
    *
-   * @param $image
+   * @param stdClass $image
    *   An image object. The $image->resource, $image->info['width'], and
    *   $image->info['height'] values will be modified by this call.
-   * @param $width
+   * @param int $width
    *   The new width of the resized image, in pixels.
-   * @param $height
+   * @param int $height
    *   The new height of the resized image, in pixels.
-   * @return
+   *
+   * @return bool
    *   TRUE or FALSE, based on success.
    *
    * @see image_resize()
@@ -51,18 +52,19 @@
   /**
    * Rotates an image the given number of degrees.
    *
-   * @param $image
+   * @param stdClass $image
    *   An image object. The $image->resource, $image->info['width'], and
    *   $image->info['height'] values will be modified by this call.
-   * @param $degrees
+   * @param int $degrees
    *   The number of (clockwise) degrees to rotate the image.
-   * @param $background
-   *   An hexadecimal integer specifying the background color to use for the
-   *   uncovered area of the image after the rotation. E.g. 0x000000 for black,
-   *   0xff00ff for magenta, and 0xffffff for white. For images that support
-   *   transparency, this will default to transparent. Otherwise it will
+   * @param string $background
+   *   (optional) An hexadecimal integer specifying the background color to use
+   *   for the uncovered area of the image after the rotation. E.g. 0x000000 for
+   *   black, 0xff00ff for magenta, and 0xffffff for white. For images that
+   *   support transparency, this will default to transparent. Otherwise it will
    *   be white.
-   * @return
+   *
+   * @return bool
    *   TRUE or FALSE, based on success.
    *
    * @see image_rotate()
@@ -72,18 +74,19 @@
   /**
    * Crops an image.
    *
-   * @param $image
+   * @param stdClass $image
    *   An image object. The $image->resource, $image->info['width'], and
    *   $image->info['height'] values will be modified by this call.
-   * @param $x
+   * @param int $x
    *   The starting x offset at which to start the crop, in pixels.
-   * @param $y
+   * @param int $y
    *   The starting y offset at which to start the crop, in pixels.
-   * @param $width
+   * @param int $width
    *   The width of the cropped area, in pixels.
-   * @param $height
+   * @param int $height
    *   The height of the cropped area, in pixels.
-   * @return
+   *
+   * @return bool
    *   TRUE or FALSE, based on success.
    *
    * @see image_crop()
@@ -96,8 +99,10 @@
    * Note that transparent GIFs loose transparency when desaturated.
    *
    * @param $image
-   *   An image object. The $image->resource value will be modified by this call.
-   * @return
+   *   An image object. The $image->resource value will be modified by this
+   *   call.
+   *
+   * @return bool
    *   TRUE or FALSE, based on success.
    *
    * @see image_desaturate()
@@ -107,9 +112,10 @@
   /**
    * Creates an image resource from a file.
    *
-   * @param $image
+   * @param stdClass $image
    *   An image object. The $image->resource value will populated by this call.
-   * @return
+   *
+   * @return bool
    *   TRUE or FALSE, based on success.
    *
    * @see image_load()
@@ -119,11 +125,12 @@
   /**
    * Writes an image resource to a destination file.
    *
-   * @param $image
+   * @param stdClass $image
    *   An image object.
-   * @param $destination
+   * @param string $destination
    *   A string file URI or path where the image should be saved.
-   * @return
+   *
+   * @return bool
    *   TRUE or FALSE, based on success.
    *
    * @see image_save()
@@ -133,9 +140,10 @@
   /**
    * Gets details about an image.
    *
-   * @param $image
+   * @param stdClass $image
    *   An image object.
-   * @return
+   *
+   * @return mixed
    *   FALSE, if the file could not be found or is not an image. Otherwise, a
    *   keyed array containing information about the image:
    *   - "width": Width, in pixels.
@@ -150,8 +158,8 @@
   /**
    * Verifies Image Toolkit is set up correctly.
    *
-   * @return
-   *   A boolean indicating if the GD toolkit is available on this machine.
+   * @return bool
+   *   True if the GD toolkit is available on this machine.
    */
   static function isAvailable();
 }
diff -u b/core/modules/system/lib/Drupal/system/Plugin/system/imagetoolkit/GDToolkit.php b/core/modules/system/lib/Drupal/system/Plugin/system/imagetoolkit/GDToolkit.php
--- b/core/modules/system/lib/Drupal/system/Plugin/system/imagetoolkit/GDToolkit.php
+++ b/core/modules/system/lib/Drupal/system/Plugin/system/imagetoolkit/GDToolkit.php
@@ -236,14 +236,14 @@
   /**
    * Creates a truecolor image preserving transparency from a provided image.
    *
-   * @param $image
+   * @param stdClass $image
    *   An image object.
-   * @param $width
+   * @param int $width
    *   The new width of the new image, in pixels.
-   * @param $height
+   * @param int $height
    *   The new height of the new image, in pixels.
    *
-   * @return
+   * @return resource
    *   A GD image handle.
    */
   public function createTmp(stdClass $image, $width, $height) {
diff -u b/core/modules/system/tests/modules/image_test/lib/Drupal/image_test/Plugin/system/imagetoolkit/TestToolkit.php b/core/modules/system/tests/modules/image_test/lib/Drupal/image_test/Plugin/system/imagetoolkit/TestToolkit.php
--- b/core/modules/system/tests/modules/image_test/lib/Drupal/image_test/Plugin/system/imagetoolkit/TestToolkit.php
+++ b/core/modules/system/tests/modules/image_test/lib/Drupal/image_test/Plugin/system/imagetoolkit/TestToolkit.php
@@ -97,10 +97,10 @@
   /**
    * Stores the values passed to a toolkit call.
    *
-   * @param $op
+   * @param string $op
    *   One of the image toolkit operations: 'get_info', 'load', 'save',
    *   'settings', 'resize', 'rotate', 'crop', 'desaturate'.
-   * @param $args
+   * @param array $args
    *   Values passed to hook.
    *
    * @see image_test_get_all_calls()