This is a tip that I've also used to great effect, but I thought that I should point out (if it isn't obvious) that you must make sure the same image always points to the same subdomain (image 1 is always linked to images1.mysite.com) otherwise you'll shoot yourself in the foot by eliminating browser caching.
I usually write a quick function in $language to turn the first few letters of the filename into ASCII then modulus by the number of fake subdomains that I've set up to decide which server to link to. The overall distribution may be imperfect but file x will always be linked to server x across pages, so it's a nice quick solution, especially if you have multiple maintainers.
I usually write a quick function in $language to turn the first few letters of the filename into ASCII then modulus by the number of fake subdomains that I've set up to decide which server to link to. The overall distribution may be imperfect but file x will always be linked to server x across pages, so it's a nice quick solution, especially if you have multiple maintainers.