UKF

I want to query this dynamic thumbnail. I want to show the previewpicture of a specific gallery ID. This works with

$gallery_id = "1";
$results = $wpdb->get_results("SELECT ng.path, np.filename, np.description, np.alttext, np.pid, np.updated_at FROM wp_ngg_pictures np, wp_ngg_gallery ng WHERE np.galleryid=ng.gid AND np.galleryid=".$gallery_id." AND np.pid=ng.previewpic",ARRAY_A);
		if(!empty($results[0]['path']) && !empty($results[0]['filename'])) :
			$imgpaththumb = $results[0]['path'].'/thumbs/thumbs_'.$results[0]['filename'];

With this query I only get the normal thumbnail which has a size of width:300. In my dynamic folder is that thumbnail with width:400. How can I manually query that dynamic thumbnail ?