What's the point of making any screenshot to prove that someone really stole something from you. First of all, if you agree to give something to someone, then you agree to a risk of losing that item. There is no such thing and no such system to make someone responsible for his action that is stealing borrowed item/poke.
If you really want to make it turn helpful instead of pointless thread, one of the tutors should be directed to list all and only players that do services such as exping other people's poke. Then, recording that person as an official 'trainer' would make him responsible for his actions of stealing. Everything would be in logs, that person x gave the "Trainer A" certain poke to level, and that person refused to give it back. Banishment would be obvious course of action against person like that and only 100 leveled players would be able to register as an official 'Trainer'. Simple as that.
Believe me nobody will make a screenshot of any kind if i'll go to a random person, or even a friend, and ask him 'hey, borrow me 1k or your pokes ill give them back in a minute'. And even if there would be a screenshot of any kind, nobody would have the right to punish me if i would get anything from a trade. Circumstances doesn't matter, trade is a trade, if you click to agree, that's over. There can be no consequences for that potential 'thief'.
On top of making a point that list of this kind is pointless, it's difficult to judge fair and square people that should be added to this list. Not to mention that it won't have that much views.