Also add the declaration of ImgPath to SingleFile.js.

lib/OpenLayers/SingleFile.js

@@ -39,5 +39,13 @@ var OpenLayers = {
             }
         }
         return (function() { return l; });
-    })()
+    })(),
+    
+    /**
+     * APIProperty: ImgPath
+     * {String} Set this to the path where control images are stored.
+     *          If set to '' OpenLayers will use script location + "img/"
+     *          Default is ''.
+     */
+    ImgPath : ''
 };