I seem to remember a couple of years ago someone whose pics had been stolen did just that - replacing them with photos of a somewhat.... errr.... interesting nature. Not quite sure how it was done, but I'm sure someone here could tell you.

I believe that was if you used photobucket and substitute the pics, but I am not sure how it would work.  I have tried it on my own listing; I put a photo in my listing from Photobucket but then noticed something I did not like, so i removed the bad pic, and tried to substitute a different photo but it did not work.
If somebody copies your photos and saves them into their computer, they have them, there is no way to "substitute" them.
The way I protect mine photos is to make them "private" in photobucket; I believe they then cannot be copied.