According to some reading I've been doing,
because browsers allow parallel downloads,
it's possible to get significant speed improvements.

e.g. if you have a PHP site.
Your includes, JS, CSS and other stuff might be pulled from

www.mydomain.com/stuff

but your images from

www.images.mydomain.com/stuff

It doesn't matter that it's on the same server.

In fact, you can have more than one subdomain but
at some point the extra HTTP requests cancel out
the benefit.

A good article.
http://tinyurl.com/23yc5hs

Of course browser allows parallel downloads, so we get nice speed improvement . But I don't see any issue to use using dedicated sub-domain for image links.
You have discussed everything so well in your post. Thanks. Keep it up.

But I don't see any issue to use using dedicated sub-domain for image links.

If you don't you won't get parallel requests happening.

Hey there.

We use a dedicated domain (not on the same server, but that is not much important) to serve images, css and javascript files.

It is a cookieless domain on an Apache 2.2 server. Gzip is activated for images/css/js files and according to Page Speed 99% of our sites have a Score of 92+ (Page Speed also depends on your own internet connection and your cache settings).

The best I could achieve for our sites is 96, but there is still something to "tweak"... hehe.

You can use subdomains, too. For example:

subdomain1.cookielessdomain.tld
subdomain2.cookielessdomain.tld
...

Now spread all your files over the cookieless subdomains and you will see in Page Speed how your Score is increasing.

Michael