Some sites use a separate domain for images the site uses, for example YouTube has ytimg.com and Twitter has twimg.com. I can see the benefits of serving the images from a separate server but why the different domain as well? Wouldn't it be simpler to just set up a subdomain like images.youtube.com to point to the server that has the images?
The benefit is that using a separate domain entirely is that it will be "cookie free", unless you set cookies for that domain of course :). Meaning less overhead on requests. If you go for a subdomain: cookies set for google.com will be passed to images.google.com as well. Micro optimization really, but makes a lot of sense if you push insane amounts of traffic, like Google.