diff --git a/core/modules/filter/filter.module b/core/modules/filter/filter.module
index 60cbd2f..b74828f 100644
--- a/core/modules/filter/filter.module
+++ b/core/modules/filter/filter.module
@@ -72,6 +72,9 @@ function filter_theme() {
     'filter_guidelines' => array(
       'variables' => array('format' => NULL),
     ),
+    'filter_html_image_secure_image' => array(
+      'variables' => array('image' => NULL),
+    ),
   );
 }
 
@@ -1778,52 +1781,70 @@ function _filter_html_escape_tips($filter, $format, $long = FALSE) {
 function _filter_html_image_secure_process($text) {
   // Find the path (e.g. '/') to Drupal root.
   $base_path = base_path();
-  $base_path_length = strlen($base_path);
+  $base_path_length = drupal_strlen($base_path);
 
   // Find the directory on the server where index.php resides.
   $local_dir = DRUPAL_ROOT . '/';
 
-  $alt_text = t('Image removed.');
-  $title_text = t('This image has been removed. For security reasons, only images from the local domain are allowed.');
-
   $html_dom = filter_dom_load($text);
   $images = $html_dom->getElementsByTagName('img');
   foreach ($images as $image) {
     $src = $image->getAttribute('src');
-    // Remove absolute URLs pointed to the local domain, in order to prevent
-    // mixed content errors.
+    // Remove absolute URLs pointing to the local domain to prevent mixed
+    // content errors.
     $image->setAttribute('src', preg_replace('|^https?://' . $_SERVER['HTTP_HOST'] . '|', '', $src));
 
-    // Verify $src starts with $base_path. This also ensures that external
-    // images can't be referenced because $base_path always starts with a
-    // slash. Another insurance against that will be prefixing the image
-    // source with the Drupal root before running getimagesize() on it.
+    // Verify that $src starts with $base_path.
+    // This also ensures that external images cannot be referenced.
     $src = $image->getAttribute('src');
-    if (substr($src, 0, $base_path_length) == $base_path) {
-      // Remove the base path() to get the path relative to the Drupal root.
-      // Also, make sure we're dealing with a real image here.
-      $local_image_path = $local_dir . substr($src, $base_path_length);
+    if (drupal_substr($src, 0, $base_path_length) === $base_path) {
+      // Remove the $base_path to get the path relative to the Drupal root.
+      // Ensure the path refers to an actual image by prefixing the image source
+      // with the Drupal root and running getimagesize() on it.
+      $local_image_path = $local_dir . drupal_substr($src, $base_path_length);
       if (@getimagesize($local_image_path)) {
-        // The image has the right path, nothing to do. Erroneous images
-        // are dealt with below.
+        // The image has the right path. Erroneous images are dealt with below.
         continue;
       }
     }
-    // Replace invalid images with an error indicator.
-    $image->setAttribute('src', $base_path . 'core/misc/message-16-error.png');
-    $image->setAttribute('alt', $alt_text);
-    $image->setAttribute('title', $title_text);
+    // Replace an invalid image with an error indicator.
+    theme('filter_html_image_secure_image', array('image' => $image));
   }
   $text = filter_dom_serialize($html_dom);
-
   return $text;
 }
 
 /**
+ * Formats an image DOM element that has an invalid source.
+ *
+ * @param DOMElement $image
+ *   An IMG node to format, parsed from the filtered text.
+ *
+ * @return void
+ *   Unlike other theme functions, the passed in $image is altered by reference.
+ *
+ * @see _filter_html_image_secure_process()
+ * @ingroup themeable
+ */
+function theme_filter_html_image_secure_image(&$variables) {
+  $image = $variables['image'];
+
+  // Turn an invalid image into an error indicator.
+  $image->setAttribute('src', base_path() . 'core/misc/message-16-error.png');
+  $image->setAttribute('alt', t('Image removed.'));
+  $image->setAttribute('title', t('This image has been removed. For security reasons, only images from the local domain are allowed.'));
+
+  // Add a CSS class to aid in styling.
+  $class = ($image->getAttribute('class') ? trim($image->getAttribute('class')) . ' ' : '');
+  $class .= 'filter-image-invalid';
+  $image->setAttribute('class', $class);
+}
+
+/**
  * Filter tips callback for secure HTML image filter.
  */
 function _filter_html_image_secure_tips($filter, $format, $long = FALSE) {
-  return t('Only images hosted on this site may be used in <img> tags. Others are replaced with an error icon.');
+  return t('Only images hosted on this site may be used in <img> tags.');
 }
 
 /**
diff --git a/core/modules/filter/lib/Drupal/filter/Tests/FilterHtmlImageSecureTest.php b/core/modules/filter/lib/Drupal/filter/Tests/FilterHtmlImageSecureTest.php
index 62fc83c..a7e38ac 100644
--- a/core/modules/filter/lib/Drupal/filter/Tests/FilterHtmlImageSecureTest.php
+++ b/core/modules/filter/lib/Drupal/filter/Tests/FilterHtmlImageSecureTest.php
@@ -63,6 +63,7 @@ function setUp() {
       'skip comment approval',
       filter_permission_name($filtered_html_format),
     ));
+    $this->drupalLogin($this->web_user);
 
     // Setup a node to comment and test on.
     $this->drupalCreateContentType(array('type' => 'page', 'name' => 'Basic page'));
@@ -75,8 +76,6 @@ function setUp() {
   function testImageSource() {
     global $base_url;
 
-    $this->drupalLogin($this->web_user);
-
     $public_files_path = variable_get('file_public_path', conf_path() . '/files');
 
     $http_base_url = preg_replace('/^https?/', 'http', $base_url);
@@ -93,7 +92,9 @@ function testImageSource() {
     $test_images = $this->drupalGetTestFiles('image');
     $test_image = $test_images[0]->filename;
 
-    // keys become the value of img src, value is what the filter should convert them to.
+    // Create a list of test image sources.
+    // The keys become the value of the IMG 'src' attribute, the values are the
+    // expected filter conversions.
     $images = array(
       $http_base_url . '/' . $druplicon => base_path() . $druplicon,
       $https_base_url . '/' . $druplicon => base_path() . $druplicon,
@@ -109,10 +110,10 @@ function testImageSource() {
     );
     $comment = array();
     foreach ($images as $image => $converted) {
-      // Include the image source for debugging.
-      $comment[] = $image;
-      // Hash image for the test attribute because it might contain characters
-      // confusing xpath.
+      // Output the image source as plain text for debugging.
+      $comment[] = $image . ':';
+      // Hash the image source in a custom test attribute, because it might
+      // contain characters that confuse XPath.
       $comment[] = '<img src="' . $image . '" testattribute="' . md5($image) . '" />';
     }
     $edit = array(
@@ -132,7 +133,7 @@ function testImageSource() {
           $this->assertEqual((string) $element['src'], $converted);
         }
       }
-      $this->assertTrue($found, t('@image was found.', array('@image' => $image)));
+      $this->assertTrue($found, format_string('@image was found.', array('@image' => $image)));
     }
   }
 }