No, you can’t — which is also the opinion of all stackoverflowers.
Nevertheless I have been experimenting a while with two interleaving images where the second picture holds only every second pixel. Swapping them really fast creates a visible picture while effectively preventing screenshots (more details at patrick-wied.at) The downsize is that even using the most advanced method like velocity.js there is flickering visible.
My second thought is more primitive – just hooking up hotkey events which is now in place here at wjst.de/images.