From http://forums.worsethanfailure.com/forums/thread/137864.aspx - "IMDB has a technique where they use CSS positioning to place a scaled 1x1 transparent gif over their photos so that right-clicking and doing a save-as saves the 1x1 gif.", I think this approach is simple and elegant

http://geekyninja.com/archives/7-great-photo-sites-for-your-blog