When it comes down to it, I don’t know if it’s worth the effort to attempt that kind of protection. The bottom line is if they can view it, they can copy it. An inclined individual could just do a print-screen to capture the image and crop it down to what they want. A screen capture is an exact pixel copy of whatever image they view, and from that point they can do whatever they want to save it back as a .jpg, .gif, or whatever.
You can try to encapsulate the image file itself in some container like a flash file or something else, but nothing can prevent a print-screen. The only way to truly protect it is to not even let them view the image unless they are authorized.
