Well nothing can stop people from stealing ideas or photoshopping out the watermark, but they are going to have to try hard. Most thieves are lazy, and the good thieves, well you are right, you cannot stop them.
If you are trying to exhibit the quality of your work though, you need to at least show the high-res detail. Art is very difficult to depict two dimensionally.
Another feature idea would be a high-res crop of the sized image.