The resuming problem is almost always on the sender's side and is related to having a router. Other than trying to help the sender fix the problem, you can't do anything about it.
Btw, resuming a jpg (even as a rar) is highly unlikely. Even a REALLY large jpg that is compressed is going to be small enough that you can download it pretty quickly, even on a modem. I think I'd try a different tactic to "fool" us into believing that the downloads are legal images and not warez.