Currently, webdiplomacy allows screenshots (implicitly, since we have no rules about it). Screenshots aren't allowed on another Diplomacy site (PlayDiplomacy), because not everyone can fake them, and this isn't Photoshop Hero, it's Diplomacy.
I've said this before, but generally, I tend to think that you shouldn't do anything that might make the moderators decide we need more rules. We're not here to edit webpages, we're here to play Diplomacy. That said, maybe if someone sends you a doctored screenshot, and it changes your mind whereas just their word wouldn't, then maybe you deserve what's coming to you.