=== modified file 'img_assist.module'
--- img_assist.module	2009-10-06 23:49:51 +0000
+++ img_assist.module	2009-10-06 23:54:07 +0000
@@ -1014,8 +1014,14 @@
   if ($regen) {
     _img_assist_build_derivatives($node, $size);
   }
-  
-  return image_display($node, $label);
+
+  // For custom size images, add a consistent CSS class, JavaScripts can rely on.
+  $attributes = array();
+  if (is_array($size) && isset($size['key']) && strpos($size['key'], 'img_assist_custom') === 0) {
+    $attributes['class'] = 'image-img_assist_custom';
+  }
+
+  return image_display($node, $label, $attributes);
 }
 
 /**