Actually I just read of a technique of using two Apache instances to ease the load on a server. One instance is barebones, the other has all the features you need for your application (PHP and any other modules/extensions) Your scripted (PHP) files will naturally only work with the one server, but your jpgs and gifs are accessed through the barebones instance. Supposedly this is easier on the server because PHP and other modules are not instantiated for each 45 byte spacer image that get's served.
The choice of which server handles the request is done via checking the file extension (or lack of one) in the apache configuration files, I wonder if that regex checking does not take more load then just serving the images?