[Eikinskjaldi]

If the attachments are on disk you will need to modify the appropriate code so that instead of just looking in the local directory it performs an ftp or scp to the remote machine for the image.

If the attacments are in a db then you can set up a new mysql handler than connects to the remote machine and uses that attachment table instead of the local one.

In either case, its a really bad idea. Massive amounts of extra overhead for minimal gain (saving a gig or so of local disk space).