I realize that my prewious answer to the original question about subdomains was a bit “off” - the poster was primerly concerned about using subdomains for branding, and I only responded with possible technical usage of subdomains because of the next to last sentence regarding serving images. So no thread hijacking here but
In a nutshell, and ignoring for a moment differences between HTTP 1.0 and HTTP 1.1, as well as difference between browsers – browsers, by default, restrict two connections per host. So if your site uses one host, all requests are forced to use two connections. It means that all of the content, images, and other objects are forced two use only two connections. Now picture a page with a lot of images (image gallery) – all of the requests for those objects will go through two connections. Idea behind multiple subdomains is to “spread” that load and allow for more then two connections. If you create CNAMEs for each subdomain, you can “trick” the browser into thinking that it is connecting to multiple servers, so if you have images1.example.com and images2.example.com and image3.example.com you would get 6 connections (vs 2) and that would increase your page load time. Note that I am not talking about improved load time for page where you have a little bit of text and nothing else, but rather page which has a lot of objects.
EDIT - PS:
To post a link or not to post a link...the link is to an interesting study so mods might let it fly - One of the Google's engineers did a study relevant to this subject - Optimizing Page Load Time [die.net]
[edited by: Tastatura at 6:31 pm (utc) on Jan. 31, 2007]