I remember seeing a free script somewhere that blocked right clicks on images and kept people from dragging and dropping them onto their desktop (I showed a while back that Serebii's script doesn't protect against drag and drop). And since it only blocks right clicks on images, then the visitor will still have access to the right click menu, just not on images. I'll have to see if I can find it, if not I can always write one.
Edit: OK, it wasn't that hard, I just threw a script together that blocks the right-click menu on images only. It also gives an alert telling the visitor why they can't right click images (instead of confusingly having the menu just not pop up with no feedback). It doesn't prevent drag and drop though, that seems to be pretty tricky to block, but at least we'll have some security.