A client (photographer) has asked me if it's possible to apply a watermark to pics on his site that show up when someone downloads them (right click - image - save as) or prints them, but NOT visible when the site is viewed in a browser. Told him I didn't hold out much hope....

I've seen on a few photo graphic sites that when you right click and image - save as is not an option. Is there a way to do this instead?
Many thanks,
LGF