I recently learned that  the http spec suggests browsers only open two simultaneous connections to a given subdomain at a time.  I haven’t tested to see how widely adopted this is.  Though I have noticed in Safari, looking at the activity viewer, that if the page requests many large assets (like a flash audio player where the user has advanced through tracks quickly), file requests start to queue up.  In a recent blog by Campaign Monitor, they mention that they server images from multiple subdomains at a time so the web browser opens up more connections at time.  You don’t even need multiple servers.  You could wildcard the subdomains on your webserver and serve up a unique subdomain.  As pointed out on Stack Overflow, you don’t want to choose a completely random subdomain on every request or the browser can’t cache the image.  It’s a pretty extreme optimization technique , though something to keep in your toolbox.