private function _cached_required()
    {
        $image_info = getimagesize($this->source_file);
        
        if (($this->url_params['w'] == $image_info[0]) and ($this->url_params['h'] == $image_info[1])) {
            $this->serve_default = TRUE;
            return FALSE;
        }
        
        return TRUE;
    }