How would I implement CDN functionality in a custom PHP-based CMS and application?

I currently manage a site that was custom developed using PHP/MySQL, and that's powered by it's own custom CMS.

Recently, we've been discussing the possibility of adapting the site so that it serves up the static assets using a CDN (ie: CSS files, JS files and images).  Since the header and footer of the site are static, it would be simple enough for us to adapt those elements so that they pull the resources from a CDN,  However, .. what about the CMS?  

In other words, .. how would we adapt the CMS so that when images are uploaded through the back end, .. that they automatically get pushed up to the CDN (using PHP) ?  To be more specific .. how would we handle the Add / Modify / Delete functionality of images within this context?  And would the switching over to the serving up of those CDN hosted assets on the front end of the site be as simple as specifying a simple "base href" tag in the site's global header?  Please advise.

Thanks!
- Yvan