diff --git a/includes/image.inc b/includes/image.inc
index e30a338..a6e7ab9 100644
--- a/includes/image.inc
+++ b/includes/image.inc
@@ -290,12 +290,12 @@ function image_resize(stdClass $image, $width, $height) {
  *   An image object returned by image_load().
  * @param $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
- *   be white.
+ * @param int $background
+ *   (optional) A 24 bit or 32 bit ARGB value specifying the background color
+ *   to use for the uncovered area of the image after the rotation (for
+ *   example, 0 for black, 16711935 for fuchsia, and 16777215 for white). For
+ *   images that support transparency, this will default to transparent.
+ *   Otherwise it will be white.
  *
  * @return
  *   TRUE on success, FALSE on failure.
@@ -431,5 +431,23 @@ function image_save(stdClass $image, $destination = NULL) {
 }
 
 /**
+ * Converts a 24 bit RGB or 32 bit ARGB value to an RGBA array.
+ *
+ * @param int $argb
+ *   The color code to convert.
+ *
+ * @return array
+ *   An array containing the values for 'red', 'green', 'blue', and 'alpha'.
+ */
+function image_dec_to_rgba($argb) {
+  return array(
+    'red' => $argb >> 16 & 0xFF,
+    'green' => $argb >> 8 & 0xFF,
+    'blue' => $argb & 0xFF,
+    'alpha' => $argb >> 24 & 0xFF,
+  );
+}
+
+/**
  * @} End of "defgroup image".
  */
diff --git a/modules/image/image.module b/modules/image/image.module
index 2122e05..dab8836 100644
--- a/modules/image/image.module
+++ b/modules/image/image.module
@@ -1413,21 +1413,3 @@ function image_filter_keyword($value, $current_pixels, $new_pixels) {
 function _image_effect_definitions_sort($a, $b) {
   return strcasecmp($a['name'], $b['name']);
 }
-
-/**
- * Converts a 24 bit RGB or 32 bit ARGB value to an RGBA array.
- *
- * @param int $argb
- *   The color code to convert.
- *
- * @return array
- *   An array containing the values for 'red', 'green', 'blue', 'alpha'.
- */
-function _image_dec_to_rgba($argb) {
-  return array(
-    'red' => $argb >> 16 & 0xFF,
-    'green' => $argb >> 8 & 0xFF,
-    'blue' => $argb & 0xFF,
-    'alpha' => $argb >> 24 & 0xFF,
-  );
-}
diff --git a/modules/simpletest/tests/image.test b/modules/simpletest/tests/image.test
index f742cda..72090c6 100644
--- a/modules/simpletest/tests/image.test
+++ b/modules/simpletest/tests/image.test
@@ -419,16 +419,13 @@ class ImageToolkitGdTestCase extends DrupalWebTestCase {
         $correct_dimensions_object = TRUE;
         $correct_colors = TRUE;
 
-        // PHP 5.5 GD bug: https://bugs.php.net/bug.php?id=65148. PHP 5.5 GD
-        // rotates differently then it did in PHP 5.4 resulting in different
-        // dimensions then what math teaches us. For the test images, the
+        // PHP 5.5 GD rotates differently than it did in PHP 5.4, resulting in
+        // different dimensions than what would normally be expected (see
+        // https://bugs.php.net/bug.php?id=65148). For the test images, the
         // dimensions will be 1 pixel smaller in both dimensions (though other
-        // tests have shown a difference of 0 to 3 pixels in both dimensions.
-        // @todo: if and when the PHP bug gets solved, add an upper limit
+        // tests have shown a difference of 0 to 3 pixels in both dimensions).
+        // @todo: If and when the PHP bug gets solved, add an upper limit
         //   version check.
-        // @todo: in [#1551686] the dimension calculations for rotation are
-        //   reworked. That issue should also check if these tests can be made
-        //   more robust.
         if (version_compare(PHP_VERSION, '5.5', '>=') && $values['function'] === 'rotate' && $values['arguments'][0] % 90 != 0) {
           $values['height']--;
           $values['width']--;
diff --git a/modules/system/image.gd.inc b/modules/system/image.gd.inc
index 28ab3f0..5bd1625 100644
--- a/modules/system/image.gd.inc
+++ b/modules/system/image.gd.inc
@@ -99,11 +99,11 @@ function image_gd_resize(stdClass $image, $width, $height) {
  * @param $degrees
  *   The number of (clockwise) degrees to rotate the image.
  * @param int $background
- *   An 24 bit or 32 bit ARGB value  specifying the background color to use for
- *   the uncovered area of the image after the rotation. E.g. 0 for black,
- *   16711935 for fuchsia, and 16777215 for white. For images that support
- *   transparency, this will default to transparent. Otherwise it will
- *   be white.
+ *   (optional) A 24 bit or 32 bit ARGB value specifying the background color
+ *   to use for the uncovered area of the image after the rotation (for
+ *   example, 0 for black, 16711935 for fuchsia, and 16777215 for white). For
+ *   images that support transparency, this will default to transparent.
+ *   Otherwise it will be white.
  * @return
  *   TRUE or FALSE, based on success.
  *
@@ -122,7 +122,7 @@ function image_gd_rotate(stdClass $image, $degrees, $background = NULL) {
   $degrees -= floor($degrees / 360) * 360;
 
   if (isset($background)) {
-    $background = _image_dec_to_rgba($background);
+    $background = image_dec_to_rgba($background);
     $background['alpha'] /= 2;
   }
   else {