Most CDNs (including Amazon’s Cloudfront, which is what I’ve used), allow origin pull, which essentially means that the CDN pulls the required entity from, in this case, your blog, the first time someone tries to access it.
If use a caching plugin like wp-supercache, or w3 total cache, they integrate with CDNs like Cloudfront to manage much of this for you. So, I can turn off my CDN by unchecking a single checkbox. This then reverts all image, js and css references back to the original location – ie your server.