Here's something I just whipped up in about 30 minutes just now.
http://www.megaxrated.com/galtest.cfm
It puts your IP address and time on the image. The location is slightly random and the text is black.
With more time I can make the text color blend in better and also be fully random anywhere in the picture. Also the information can be anything, IP, userid, some unique code that the user doesn't know, etc.
It's done with ColdFusion. Woo.
I used some Karups pics for the test because I already had a gallery built on my server with easy access to the images.
Also I would install a pixel font on my server for the smallest readable type.
A surfer could erase the info, but it would definitely be a pain in their ass.