prolly not the best way, but it would work ....
on your gallery, add a 1x1 image and img src your own tgp (dont lie, you know you have one)
add your gallery url as a trade and disable it.
every hit you get to gallery should register a hit on your tgp from that disabled trade.
now every now and then check referring url for that disabled trade ...
it will show a list of all the gallery mirrors recieving traffic.
IF there were any stolen ones, it would be on a different domain, and you can go there to see the recips and where it is listed, note the domain thats used, come here and expose him.
oh god i cant wait to see the replies on this one .... lets here it fletch