Sadly, there really is no absolute way to do this. You could google "disable right click javascript" and you will find a few ways....but people can also find directions on how to disable that js. I have looked and looked in the past for a way to do just what you are needing...but never found a reliable way. Some people put their image as a background in a table, then put a transparent gif image over it....that helps..but you can still get around that. If someone really wants it, they can always find a way...be it through screen shots, or snagit, snipping tool...there are just tons of things to help people find ways around any security you may implement. Sorry for the bad news, but maybe someone will come out with something...who knows.
