Plus, it's a very minor issue. Games are not going to be won or lost based on screenshot trades. Even if you learn the location of another civ or find out that someone doesn't have a particular resource, that's something that could just as easily have been conveyed in text (e.g. "Team X is south of you", or "Team Y doesn't have Copper connected").
I think it's far simpler to just allow teams to trade screenshots if they so desire, rather than potentially forcing admin interventions on a fairly insignificant and trivial matter. Teams will be naturally cautious about trading screenshots anyway - and even if they're extremely liberal with trading them, it won't yield them any significant advantage.
Overall it doesn't seem to make sense to make a rule banning the relatively unimportant trading of screenshots, which can be circumvented anyway by diplomacy in a huge number of ways with a huge amount of grey area which isn't really enforceable.
Actually most of that is the reasoning I'm using to suggest that we should have a rule limiting screenshot trading

It is a fairly insignificant issues in terms of the outcome of the game, there are a lot of ways around it, with verbal descriptions, and yes it is unenforceable. I don't think anyone has argued that screenshot trading should be banned because it's unfair or game breaking, and that we need an ironclad rule to prevent it. I think it should be banned because the alternative, discussing the map verbally, having a little more uncertainty, and a longer exploration phase is much more fun.
The rule does not need to be enforced, if someone trades an ASCI map, or even a real screen shot they aren't going to gain a significant advantage because of it. It's not something that I'd think is worth going to an admin over. I believe that an issue this simple can be handled by the honor system, with a minimum of violations.
I expect most people will obey the rule out of their own self interest, not wanting to give too many details about their own land, or land they want to settle. The rest will obey the rule because it takes two to map trade, and I doubt you'll want to approach another team with a suggestion that they break the rules with you, even if it's a minor one. That's not a good way to gain a trusting ally.