Try this plugin:
https://swearingscience.com/cdn-tools/
So far it only works with Cloud Files CDN but others are coming
From the site:
* One click load all attachments and JS to a CDN
* Transparently sideload new content (including images resized by WP!)
* Automatically rewrite URLs on your blog to point at the CDN-loaded content.
* Easy deactivation if you decide to stop using it. No messy fixes to get back to a pristine WP install. Just remove the plugin and everything will work fine!
