This is a common problem I have had to deal with in the recent weeks, clients from projects that I have worked in the past have asked me to help them protect the images that they have on their websites and even after explaining them that once their images/content are on the internet the only thing that is possible is that we can make it harder for people to steal their content but we cant stop them from doing that. Another related project which I recently faced during the devlopment of http://gallery.search.yahoo.com was that the monkey preview was actually an iframe which contained actual links and for obvious reasons we didnt want people clicking on the links inside the iframe.
The solution that was applied to solve both the problem was a fairly straight forward one, the idea was to stretch a 1px transparent gif/png over the image or the iframe I didnt want people to right click on and even if they tried to do that, they would only be able to save the 1px gif instead of the actual image below it. As for the iframe, since it would be below the image, the links will not be click able. I had initially seen this being used on flickr and thought it was quite cool, though it might mess upsemantic markup a bit. The alt tag for this extra bit of image should not be empty here I suppose as its suppose to relay the same kind of information as the image/iframe it trying to mask however the contents of the alt tag should not be repeated.
