I'm very very lost when it comes to CDN. I'm looking

Hi, I'm very very lost when it comes to CDN.

I'm looking at MaxCDN and talked to support. I need to find out if my subsites (not subdomains) 3dartistnetwork.com/users all upload to one media folder or do they have seperate folders.

Each of my sites have their own marketplace theme and handle their own payments. Many add their own images for slideshows. etc ...

Do they all upload to the same media folder. Or do you have any suggestions on how to handle my CDN issue? I'm running out of room on my web hosting which is causing errors.

UPDATE: I found where everything goes into one sites folder. Any thoughts on how to handle that with a CDN.

BTW: The W3 Total Cache totally crashed my site when I used it. So that option is totally out.

Back Again: I have WP SUPER CACHE and found http://support.maxcdn.com/pullzone/wordpress-wp-super-cache/. Would that cover the whole network since I'm setting it up on the network admin?

Thanks
Kimberly