Image theft

A fellow photographer wrote to me expressing concern that his photographs and mine are being used without permission on Ebay. The link is here. I’d like to take action to resolve this, as image theft is a rampant problem throughout the ‘net. I realize there’s next to nothing I can do to stop people from stealing my photos without permission once I post them on the ‘net (I never post full size photos), but I’d like to think I can start somewhere.

This certainly isn’t the first time I’ve seen this happen. Does anyone have thoughts on how I may resolve this particular case?