The only real problem I can think of is that the file is inherently being downloaded to the viewer's computer when they simply look at the picture.
You can do all sorts of tricks (like the invisible gif or messing with the right click menu) to hobble most normal viewers from grabbing the full-size image but those are easily gotten around by anyone with experience (or the ability to effectively use Google). There might be a way to cut off viewing the full size images, somehow, but a little research and investigation would let a person go straight to the image via typing in the URL (after all, your thumbs are a standard renaming scheme and a standard location).
You might try something where you force everyone to log in (and can't see anything or only certain albums until they do) and then cut them off if they don't post so often/so much/at all.