Seperate Drupal instance as files server for other Drupal instances
You could use feeds module to periodically import file URIs from XML on your file server to you individual drupal instances. You might then be able to hook that up with the media module and use the files on all your different sites. 

Edit:
If you're just looking for a cdn for your images you could probably just use Amazon S3. Looks like there's a few modules already built to work with it