If you've got access to your .htaccess file, pseudo-subdomains can easily solve this problem for you. Using a bit of PHP and some .htaccess mod_rewrites, you can easily redirect absolutely anything you want to your images directory. Using this logic, you can set some scripts up to automatically parallelize images, js, etc. for you, dynamically. For example, every X references to images, js, etc. it will automatically switch to another domain. Or, every Y kilobytes. However you want your website(s) to automatically switch to an unlimited number of subdomains.
I've created an example of this structure, and you can even generate your own URL on my website, allowing the absolute easiest solution to parallelization without coding or mod_rewrites. Feel free to take a look.