upload files to a different server

eventually one of my friends websites is going to have a lot of traffic so im trying to get his site prepared for the future, im wanting to store pictures on a different server but i want the functionality of cropping auto make thumbnails water marking etc..
but i dont see a ready way to, through the main site, upload say a, featured picture.. which we use, and uploading it to another server.. is there any articles about this anyone could point to?
Im assuming its somthing like a php script, on the other server, that is setup to communicate with the main server.. any help is appreciated.
Thanks!