I'd like to offer another, simpler alternative. If the goal is simply to prevent people from linking directly to your images, why not use regular HTML tags... to temporary image files? It's very easy to move files around or rename them.

The best solution I've seen is to create symbolic links that you then destroy at some later time, either during your page exit Javascript, your next page CGI from an action button, or as a recurring housecleaning function. Check out symlink in your Perl docs. Caveat: If your web system is Doze-hosted, you'll have to move or rename files; symlinking doesn't work.