CloudFront is pretty good, just make sure you are able to config your asset source in one line. Otherwise you have to use a tool to invalidate the cloudfront cache frequently during dev and it's not instant.
I've used the dropbox API before to automatically upload photos in a dropbox folder to Flickr. It occurs on an interval (cron job every 2 mins). I'm sure you could do the same thing using FTP or a custom API that exists on your destination server.