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/lib/Drupal/Core/ImageToolkit/ImageToolkitInterface.php b/core/lib/Drupal/Core/ImageToolkit/ImageToolkitInterface.php
index 8960504..bf219b1 100644
--- a/core/lib/Drupal/Core/ImageToolkit/ImageToolkitInterface.php
+++ b/core/lib/Drupal/Core/ImageToolkit/ImageToolkitInterface.php
@@ -159,6 +159,15 @@ public static function isAvailable();
   public static function getSupportedExtensions();
 
   /**
+   * Returns a list of image types supported by the toolkit.
+   *
+   * @return array
+   *   An array of available image types. An image type is represented by a PHP
+   *   IMAGETYPE_* constant (e.g. IMAGETYPE_JPEG, IMAGETYPE_PNG, etc.).
+   */
+  public static function supportedTypes();
+
+  /**
    * Applies a toolkit operation to an image.
    *
    * @param string $operation
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/system/src/Plugin/ImageToolkit/GDToolkit.php b/core/modules/system/src/Plugin/ImageToolkit/GDToolkit.php
index 98ae863..70aa477 100644
--- a/core/modules/system/src/Plugin/ImageToolkit/GDToolkit.php
+++ b/core/modules/system/src/Plugin/ImageToolkit/GDToolkit.php
@@ -369,13 +369,9 @@ public static function getSupportedExtensions() {
   }
 
   /**
-   * Returns a list of image types supported by the toolkit.
-   *
-   * @return array
-   *   An array of available image types. An image type is represented by a PHP
-   *   IMAGETYPE_* constant (e.g. IMAGETYPE_JPEG, IMAGETYPE_PNG, etc.).
+   * {@inheritdoc}
    */
-  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..562ac16 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 in the specified format.")
  * )
  */
 class Convert extends GDImageToolkitOperationBase {
@@ -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['format']) {
         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..91cb065 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.
@@ -180,6 +180,18 @@ function testManipulations() {
         'height' => 20,
         'arguments' => array('format' => 'jpeg'),
       ),
+      'convert' => array(
+        'function' => 'convert',
+        'width' => 40,
+        'height' => 20,
+        'arguments' => array('format' => 'gif'),
+      ),
+      'convert' => array(
+        'function' => 'convert',
+        'width' => 40,
+        'height' => 20,
+        'arguments' => array('format' => 'png'),
+      ),
     );
 
     // Systems using non-bundled GD2 don't have imagerotate. Test if available.
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);
   }