The only thing that would do is slow down someone trying to save the pics and annoy the regular members just trying to navigate this site. Flickr has a method of preventing people from downloading "protected" photos but even then if someone really wants a copy they can just hit print screen.
Watermarks could be somewhat effective in at least letting someone know the origin of the pic and that you dont want it reposted, but unless they're plastered across the center of every pic (something I personally find annoying) they can just be cropped/photoshopped out.
A combination of a watermark and restricting competition access to platinum members would probably be the most effective way of preventing the pics from being reposted simply because the amount of people with platinum memberships is significantly smaller than the number of free members. But removing the competition from the free section might reduce the amount of people (like myself) who signed up for a platinum membership after seeing some of the pics that the members were posting every month.
its a tough call, freedom vs security. In all honesty this is making me a little more sympathetic (not that much) to the RIAA & MPAA's use of DRM.