Here's another thumbs up for a second, custom httpd process. A few years ago one of my sites had some severe overload problems, compiling a "lean and mean" Apache with most modules stripped out (including many of the default ones) for serving IMAGES ONLY made a huge difference in server load. It will also make a difference in your memory usage, standard Apache with PHP consumed about 7Mb, the lean Apache consumed 4Mb. Multiply that by 100 or 200 httpd processes and you get back a sizeable chunk of RAM. Most TGPs probably spend most of their server time sending images, so it is well worth considering the benefits.
If you run templates that can rebuild your site with img src's pointing to a different domain then you can set the whole thing up in less than an hour.