Do what I do, create an image with the URL to your site on it. Then setup mod_rewrite to point every direct link that doesnt come from your domain to show this image.
Free advertising. Whenever he gets someone to the gallery it shows an image in every single spot that says "Come to
www.mydomain.com -- more free porn" or something along those lines. Since he is using your bandwidth, make it useful at least.
AJ