diff --git a/includes/theme.inc b/includes/theme.inc
index 6c2b640..dc6c821 100644
--- a/includes/theme.inc
+++ b/includes/theme.inc
@@ -1512,9 +1512,17 @@ function theme_image($variables) {
   $attributes['src'] = file_create_url($variables['path']);
 
   foreach (array('width', 'height', 'alt', 'title') as $key) {
-
-    if (isset($variables[$key])) {
-      $attributes[$key] = $variables[$key];
+    // At least 'alt' defaults to an empty string; allow to omit an attribute by
+    // overriding the variable to NULL.
+    if (array_key_exists($key, $variables)) {
+      // If there is a value for the variable, it overrides the attribute.
+      if (isset($variables[$key])) {
+        $attributes[$key] = $variables[$key];
+      }
+      // Otherwise, remove the attribute.
+      else {
+        unset($attributes[$key]);
+      }
     }
   }