Just like any kind of security, the best you can do, really, is throw up virtual "roadblocks". The truly committed thief will not be deterred no matter what, time is on his/her side and all the tools required to get the job done are out there and free for the downloading.
I mean, in all honesty, if I decide I want your shot, with a few clicks of my mouse (well, six, actually) I can download the entire WEB PAGE, where you host your photo(s), discard those elements I don't want and be on my merry way with your pictures.
As much as it sucks, the cold, hard fact of the matter is... If we post it, we are potentially giving it away. Period. End of discussion.