This article is from the eBay FAQ, by William B. Shaw firstname.lastname@example.org with numerous contributions by others.
If they are linking to a picture in your webspace, you can always replace
the picture. A great gag would be to use a picture of your item, but
horrible destroyed. Be sure none of your current auctions are using the same
picture! If the seller was smart enough to copy your picture before using
it, you can complain to ebay or the seller. This is mostly ineffective since
eBay takes 2-3 days to respond and you'll need to prove somehow that you
indeed own the picture. If you consider your artwork and auction listings
valuable, register with eBay's VeRO program and state your rights on About
Me page. I haven't heard any feedback on how effective this tactic is, but
the VeRO email address is looked at quicker than safeharbor.
A good way to discourage people from using your pictures is to put your name
or eBay handle in the graphic image with an editor. For example, put the
text "Copyright 2001 -your ebay handle-" in the image using your favorite
paint program before posting it. Someone could always edit it back out, but
it takes much more effort, especially if your background is multi-colored.