I right clicked on everything on your site, I would suggest another right click script and definately ban that IP. I think they are interested in your graphics, it is a nice site with lots to offer for someone looking for that type of pictures.
Also, contact the owner of the site and say, "Look, if you want to use the pictures that is fine, but don't link to my site to do it, download them and then upload them to your own server." It sounds to me like they have just linked to your pictures and everytime someone loads there site it counts as a hit to yours.
Anyway, best of luck to you. I wish I had someone that interested in any of my 30+ sites.
I think you are really onto something, although it looks like the main objective was not really to use up some of your bandwidth, but rather to manipulate your site's usage log to create what appear to be extra links to their site. That is, to make search engines think that your site actually contains links to theirs, to help move their site up in Google (etc.) search results for keywords that appear on their site.
A Google search for "referer spam" yields some interesting info, such as:
It looks like part of the problem is if sites around the Internet have their access logs available to be viewed by anyone, this can make them targets for this sort of spam.
The wikipedia article I linked above explains the problem, and outlines some solutions.
