unfortunatly this protection can be with minor efforts. lets say 15 sec. of extra time and you have the desired picture on your pc.
so while i don't know how the admin would think about babbling this workaround, i will keep it a secret
i also don't think, that you can asure complete safety for all graphics oin a webpage. you can just keep the average user from downloading them on the common way with the tools the browser supplies....