Generally, the risk of what happens in shipment of goods is the responsibility of the recipient. But, usually the recipient is paying for the shipping, and has the option of adding insurance or tracking to reduce the risk, but this is not the case in our exchanges.
I think it might be resonable to request that people sending prints add whatever tracking service is available. For U.S. domestic mail, delivery confirmation is only $0.65, and I think it's safe to assume that the sender values his print by at least that much.
The tracking doesn't guarantee delivery, but at least the recipient, or the sender has a way of getting the Post to figure out what happened.