Yootheme Zoo İmage Watermark

I have use to yootheme zoo applications. I need to watermark at zoo images.

http://ift.tt/18xh4Wf this is zoo imege progress file

and i find this code

@imagecopy($thumbnail, $src, 0, 0, $src_x, $src_y, $this->thumb_width, $this->thumb_height);

Now how can i watermark from this file ?