Trading photos is a pain. There's always a debate about who is going to send theirs first. There should be an escrow option to send a photo but that photo only gets unlocked to the recipient if they have put a photo in escrow too. Both photos unlock simultaneously. Fair trade.

The notification could say, "this user has chosen photo escrow to trade photos. If you wish to unlock send a photo too and both will unlock simultaneously.”

This would reduce abuse in photo soliciting and trading, which causes strong negative experience in users that fall victims to photo abuse.