$15.00 is NOTHING!!!!!! I would send a very very stern letter saying that you own the copyright to the images that they are using and that they are digitally watermarked. Let them know that you would gladly license them the rights to display your images for a usage fee of $500.00 per image, otherwise please remove them!
The following excerpts (which you might just mention...) are from the book, Content Rights For Creative Professionals; Copyrights and Trademarks in a Digital Age. 2nd Edition, Focal Press.
"Under copyright rules if someone copies, distributes, or displays a copyrighted work publicly without authority of the copyright owner or its agent, then a violation of the law has occurred."
"To ensure that the copyright owner is compensated when works are infringed on, the law establishes statuatory damages, which may be sought in lieu of the others. Statuatory damages can be awarded in the range of $750.00 to $30,000 per work infringed; if the actions of the infringer are determined to be willful, these claims can baloon to $150,000 per work."
Here's a true copyright story. One time we created 3D animations for "video" for a client. Then it turns out, that some of the still images from the animations were published in a book (by a HUGE unrelated company). We had to 'educate' the publishers that we still owned the copyright to the work for print. They then tried to get us to 'quickly' sign a form giving them the rights to print the images. We said that we would license them the usage rights for "a fee". The fee actually doubled because the book was about to be released in both the Americas and Europe. They had a few more folks call at various times tring to just get us to ' just sign ' the form. Nope. End of story: we made more $$ off licensing the rights to the images than for the original 3D animation work!!
Oh yeah, great images by the way!
WildBill