KimmyKim, there are a lot of moving parts in what you described and I would venture to guess that there is no plugin available for it. I would say the easiest way to do it is to just have them right-click and save the image, then upload to their own media library.
The plugin would have to access your library, offer them an import option, then transfer the image from your WP library to theirs. That's not as easy as it sounds. Personally, I would create a portfolio that displays the images and an option to download. Then they can insert it into a post which automatically uploads it to their library. And if they don't know how to insert a photo into a post in WP, then you should probably take their website away from them.
