commit 065b17bedb44dd993534ee83253ac11589acaa94
Author: Joel Pittet <joel@pittet.ca>
Date:   Fri Aug 29 14:03:48 2014 -0700

    message

diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 2bdfff5..11a20b9 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -1124,11 +1124,12 @@ function template_preprocess_image(&$variables) {
   $variables['attributes']['src'] = file_create_url($variables['uri']);
 
   foreach (array('width', 'height', 'alt', 'title') as $key) {
-    // If the property has already been defined in the attributes, do not override.
-    if (array_key_exists($key, $variables['attributes'])) {
-      continue;
-    }
     if (isset($variables[$key])) {
+      // If the property has already been defined in the attributes,
+      // do not override, including NULL.
+      if (isset($variables['attributes'][$key]) || array_key_exists($key, $variables['attributes'])) {
+        continue;
+      }
       $variables['attributes'][$key] = $variables[$key];
     }
   }