diff --git a/core/lib/Drupal/Core/Image/ImageInterface.php b/core/lib/Drupal/Core/Image/ImageInterface.php
index b8ca6a8..e0b6bda 100644
--- a/core/lib/Drupal/Core/Image/ImageInterface.php
+++ b/core/lib/Drupal/Core/Image/ImageInterface.php
@@ -149,13 +149,16 @@ public function scale($width, $height = NULL, $upscale = FALSE);
   public function scaleAndCrop($width, $height);
 
   /**
-   * Converts an image to a specified format.
+   * Instructs the toolkit to save the image in the specified format.
    *
    * @param string $format
    *   The format to convert to (e.g. 'jpeg', 'png'). Allowed values depend on
    *   the implementation of the convert operation of the image toolkit. For a
    *   working example, see
    *   \Drupal\system\Plugin\ImageToolkit\Operation\gd\Convert.
+   *
+   * @return bool
+   *   TRUE on success, FALSE on failure.
    */
   public function convert($format);
 
diff --git a/core/modules/image/src/Controller/ImageStyleDownloadController.php b/core/modules/image/src/Controller/ImageStyleDownloadController.php
index 173114e..2b2d89f 100644
--- a/core/modules/image/src/Controller/ImageStyleDownloadController.php
+++ b/core/modules/image/src/Controller/ImageStyleDownloadController.php
@@ -135,11 +135,15 @@ public function deliver(Request $request, $scheme, ImageStyleInterface $image_st
       // the actual source image, we remove the extension and check if that
       // image exists.
       $path_info = pathinfo($image_uri);
-      $image_uri = $path_info['dirname'] . DIRECTORY_SEPARATOR . $path_info['filename'];
-      if (!file_exists($image_uri)) {
+      $converted_image_uri = $path_info['dirname'] . DIRECTORY_SEPARATOR . $path_info['filename'];
+      if (!file_exists($converted_image_uri)) {
         $this->logger->notice('Source image at %source_image_path not found while trying to generate derivative image at %derivative_path.',  array('%source_image_path' => $image_uri, '%derivative_path' => $derivative_uri));
         return new Response($this->t('Error generating image, missing source file.'), 404);
       }
+      else {
+        // The converted file does exist, use it as the source.
+        $image_uri = $converted_image_uri;
+      }
     }
 
     // Don't start generating the image if the derivative already exists or if
diff --git a/core/modules/image/src/Plugin/ImageEffect/ConvertImageEffect.php b/core/modules/image/src/Plugin/ImageEffect/ConvertImageEffect.php
index 5091e51..50ceed3 100644
--- a/core/modules/image/src/Plugin/ImageEffect/ConvertImageEffect.php
+++ b/core/modules/image/src/Plugin/ImageEffect/ConvertImageEffect.php
@@ -18,7 +18,7 @@
  * @ImageEffect(
  *   id = "image_convert",
  *   label = @Translation("Convert"),
- *   description = @Translation("Converts an image between formats (e.g. from PNG to JPEG).")
+ *   description = @Translation("Converts an image between extensions (e.g. from PNG to JPEG).")
  * )
  */
 class ConvertImageEffect extends ConfigurableImageEffectBase {
@@ -33,7 +33,7 @@ public function transformDimensions(array &$dimensions) {
    * {@inheritdoc}
    */
   public function applyEffect(ImageInterface $image) {
-    if (!$image->convert($this->configuration['format'])) {
+    if (!$image->convert($this->configuration['extension'])) {
       $this->logger->error('Image convert failed using the %toolkit toolkit on %path (%mimetype)', array('%toolkit' => $image->getToolkitId(), '%path' => $image->getSource(), '%mimetype' => $image->getMimeType()));
       return FALSE;
     }
@@ -46,14 +46,14 @@ public function applyEffect(ImageInterface $image) {
   public function getDerivativeMimeType(&$mime_type) {
     // The guess method on the MIME type guesser needs a full filename. It does
     // not work with just an extension.
-    $mime_type = \Drupal::service('file.mime_type.guesser.extension')->guess('test.' . $this->configuration['format']);
+    $mime_type = \Drupal::service('file.mime_type.guesser.extension')->guess('test.' . $this->configuration['extension']);
   }
 
   /**
    * {@inheritdoc}
    */
   public function getDerivativeExtension(&$extension) {
-    $extension = $this->configuration['format'];
+    $extension = $this->configuration['extension'];
   }
 
   /**
@@ -61,7 +61,7 @@ public function getDerivativeExtension(&$extension) {
    */
   public function getSummary() {
     $summary = array(
-      '#markup' => Unicode::strtoupper($this->configuration['format']),
+      '#markup' => Unicode::strtoupper($this->configuration['extension']),
     );
     $summary += parent::getSummary();
 
@@ -73,7 +73,7 @@ public function getSummary() {
    */
   public function defaultConfiguration() {
     return array(
-      'format' => NULL,
+      'extension' => NULL,
     );
   }
 
@@ -86,10 +86,10 @@ public function buildConfigurationForm(array $form, FormStateInterface $form_sta
       $extensions,
       array_map(array('\Drupal\Component\Utility\Unicode', 'strtoupper'), $extensions)
     );
-    $form['format'] = array(
+    $form['extension'] = array(
       '#type' => 'select',
-      '#title' => t('Format'),
-      '#default_value' => $this->configuration['format'],
+      '#title' => t('Extension'),
+      '#default_value' => $this->configuration['extension'],
       '#required' => TRUE,
       '#options' => $options,
     );
@@ -101,7 +101,7 @@ public function buildConfigurationForm(array $form, FormStateInterface $form_sta
    */
   public function submitConfigurationForm(array &$form, FormStateInterface $form_state) {
     parent::submitConfigurationForm($form, $form_state);
-    $this->configuration['format'] = $form_state->getValue('format');
+    $this->configuration['extension'] = $form_state->getValue('extension');
   }
 
 }
diff --git a/core/modules/image/src/Tests/ImageEffectsTest.php b/core/modules/image/src/Tests/ImageEffectsTest.php
index 609c356..1549d5b 100644
--- a/core/modules/image/src/Tests/ImageEffectsTest.php
+++ b/core/modules/image/src/Tests/ImageEffectsTest.php
@@ -92,14 +92,35 @@ function testCropEffect() {
    * Test the image_convert_effect() function.
    */
   function testConvertEffect() {
+    // Test jpg.
     $this->assertImageEffect('image_convert', array(
-      'format' => 'png',
+      'extension' => 'jpg',
     ));
     $this->assertToolkitOperationsCalled(array('convert'));
 
     // Check the parameters.
     $calls = $this->imageTestGetAllCalls();
-    $this->assertEqual($calls['convert'][0][0], 'png', 'Format was passed correctly');
+    $this->assertEqual($calls['convert'][0][0], 'jpg', 'Extension was passed correctly');
+
+    // Test gif.
+    $this->assertImageEffect('image_convert', array(
+      'extension' => 'gif',
+    ));
+    $this->assertToolkitOperationsCalled(array('convert'));
+
+    // Check the parameters.
+    $calls = $this->imageTestGetAllCalls();
+    $this->assertEqual($calls['convert'][0][0], 'gif', 'Extension was passed correctly');
+
+    // Test png.
+    $this->assertImageEffect('image_convert', array(
+      'extension' => 'png',
+    ));
+    $this->assertToolkitOperationsCalled(array('convert'));
+
+    // Check the parameters.
+    $calls = $this->imageTestGetAllCalls();
+    $this->assertEqual($calls['convert'][0][0], 'png', 'Extension was passed correctly');
   }
 
   /**
diff --git a/core/modules/system/src/Plugin/ImageToolkit/GDToolkit.php b/core/modules/system/src/Plugin/ImageToolkit/GDToolkit.php
index 98ae863..14ee446 100644
--- a/core/modules/system/src/Plugin/ImageToolkit/GDToolkit.php
+++ b/core/modules/system/src/Plugin/ImageToolkit/GDToolkit.php
@@ -375,7 +375,7 @@ public static function getSupportedExtensions() {
    *   An array of available image types. An image type is represented by a PHP
    *   IMAGETYPE_* constant (e.g. IMAGETYPE_JPEG, IMAGETYPE_PNG, etc.).
    */
-  protected static function supportedTypes() {
+  public static function supportedTypes() {
     return array(IMAGETYPE_PNG, IMAGETYPE_JPEG, IMAGETYPE_GIF);
   }
 }
diff --git a/core/modules/system/src/Plugin/ImageToolkit/Operation/gd/Convert.php b/core/modules/system/src/Plugin/ImageToolkit/Operation/gd/Convert.php
index 38dfff8..d9c591e 100644
--- a/core/modules/system/src/Plugin/ImageToolkit/Operation/gd/Convert.php
+++ b/core/modules/system/src/Plugin/ImageToolkit/Operation/gd/Convert.php
@@ -17,7 +17,7 @@
  *   toolkit = "gd",
  *   operation = "convert",
  *   label = @Translation("Convert"),
- *   description = @Translation("Converts an image to the given format.")
+ *   description = @Translation("Instructs the toolkit to save the image with the specified extension.")
  * )
  */
 class Convert extends GDImageToolkitOperationBase {
@@ -27,8 +27,8 @@ class Convert extends GDImageToolkitOperationBase {
    */
   protected function arguments() {
     return array(
-      'format' => array(
-        'description' => 'The new format of the converted image',
+      'extension' => array(
+        'description' => 'The new extension of the converted image',
       ),
     );
   }
@@ -37,8 +37,8 @@ protected function arguments() {
    * {@inheritdoc}
    */
   protected function validateArguments(array $arguments) {
-    if (!in_array($arguments['format'], $this->getToolkit()->getSupportedExtensions())) {
-      throw new \InvalidArgumentException(String::format("Invalid format (@value) specified for the image 'convert' operation", array('@value' => $arguments['format'])));
+    if (!in_array($arguments['extension'], $this->getToolkit()->getSupportedExtensions())) {
+      throw new \InvalidArgumentException(String::format("Invalid extension (@value) specified for the image 'convert' operation", array('@value' => $arguments['extension'])));
     }
     return $arguments;
   }
@@ -47,24 +47,12 @@ protected function validateArguments(array $arguments) {
    * {@inheritdoc}
    */
   protected function execute(array $arguments = array()) {
+    // Make sure we have an image type.
     $type = FALSE;
-    switch ($arguments['format']) {
-      case 'jpeg':
-      case 'jpg':
-      case 'jpe':
-        $type = IMAGETYPE_JPEG;
+    foreach ($this->getToolkit()->supportedTypes() as $type) {
+      if (image_type_to_extension($type) === $arguments['extension']) {
         break;
-
-      default:
-        $constant = 'IMAGETYPE_' . \Drupal\Component\Utility\Unicode::strtoupper($arguments['format']);
-        if (defined($constant)) {
-          $type = constant($constant);
-        }
-        break;
-    }
-
-    if (!$type) {
-      return FALSE;
+      }
     }
 
     $res = $this->getToolkit()->createTmp($type, $this->getToolkit()->getWidth(), $this->getToolkit()->getHeight());
@@ -74,8 +62,8 @@ protected function execute(array $arguments = array()) {
 
     imagedestroy($this->getToolkit()->getResource());
 
+    // Update the image object.
     $this->getToolkit()->setType($type);
-    // Update image object.
     $this->getToolkit()->setResource($res);
 
     return TRUE;
diff --git a/core/modules/system/src/Tests/Image/ToolkitGdTest.php b/core/modules/system/src/Tests/Image/ToolkitGdTest.php
index 93cc6ae..40a8691 100644
--- a/core/modules/system/src/Tests/Image/ToolkitGdTest.php
+++ b/core/modules/system/src/Tests/Image/ToolkitGdTest.php
@@ -8,7 +8,7 @@
 namespace Drupal\system\Tests\Image;
 
 use Drupal\Core\Image\ImageInterface;
-use Drupal\simpletest\DrupalUnitTestBase;
+use \Drupal\simpletest\KernelTestBase;
 use Drupal\Component\Utility\String;
 
 /**
@@ -17,7 +17,7 @@
  *
  * @group Image
  */
-class ToolkitGdTest extends DrupalUnitTestBase {
+class ToolkitGdTest extends KernelTestBase {
 
   /**
    * The image factory service.
@@ -174,11 +174,26 @@ function testManipulations() {
         'height' => 8,
         'corners' => array_fill(0, 4, $this->black),
       ),
-      'convert' => array(
+      'convert_jpg' => array(
         'function' => 'convert',
         'width' => 40,
         'height' => 20,
-        'arguments' => array('format' => 'jpeg'),
+        'arguments' => array('extension' => 'jpeg'),
+        'corners' => $default_corners,
+      ),
+      'convert_gif' => array(
+        'function' => 'convert',
+        'width' => 40,
+        'height' => 20,
+        'arguments' => array('extension' => 'gif'),
+        'corners' => $default_corners,
+      ),
+      'convert_png' => array(
+        'function' => 'convert',
+        'width' => 40,
+        'height' => 20,
+        'arguments' => array('extension' => 'png'),
+        'corners' => $default_corners,
       ),
     );
 
diff --git a/core/modules/system/tests/modules/image_test/src/Plugin/ImageToolkit/TestToolkit.php b/core/modules/system/tests/modules/image_test/src/Plugin/ImageToolkit/TestToolkit.php
index f72eaaa..b1b31a7 100644
--- a/core/modules/system/tests/modules/image_test/src/Plugin/ImageToolkit/TestToolkit.php
+++ b/core/modules/system/tests/modules/image_test/src/Plugin/ImageToolkit/TestToolkit.php
@@ -255,7 +255,7 @@ public static function getSupportedExtensions() {
    *   An array of available image types. An image type is represented by a PHP
    *   IMAGETYPE_* constant (e.g. IMAGETYPE_JPEG, IMAGETYPE_PNG, etc.).
    */
-  protected static function supportedTypes() {
+  public static function supportedTypes() {
     return array(IMAGETYPE_PNG, IMAGETYPE_JPEG, IMAGETYPE_GIF);
   }