I like the idea.
Of course people can easily remove the watermark. But it's better than nothing.
Here is what I came up with:
http://sexteri.com/watermark/pic.jpg
It uses a php wrapper. This solution also seems to be faster than the previously posted one.
It's very easy to program an on-the-fly watermark. I'm not even a programmer. I use this solution to watermark all the images on one of my blogs.
I think it should be no problem to program a little application that adds the user to a database after signup. CCBill offers the post variables for this purpose. On your site, you could create a session for the user when he logs in and then serve the images on the fly to him.