serving all images through a file

Hello collective mind, I'm quoting and experimenting with some code for a site which includes a 5 level permission system for the images (no prize for guessing it's an "adult" related site). Anyway, I'm toying with the idea that all images in a certain directory will be blocked via a .htaccess file so that they can't be served by Apache. Then I'll need a small PHP script to validate the user and permission level, then pass the images through, which I've done before without any trouble. However, the *theory* is that this site will generate a LOT of traffic, and I'm a little concerned about the big picture... Will forcing all images through this image script cause a bottle neck on the site, or do anything stupid like triple server load? TIA, Justin