If you are an idealist, and don't want to cheat and don't want others to cheat in a PBEM game, here's a suggestion:
Ask everyone playing your PBEM game whether THEY think a certain action (e.g. trading screenshots) is cheating.

What will result is either:
a) There will be consensus that it IS cheating, is deplorable, carries penalties when caught, etc., and you'll know where you stand - i.e. don't cheat or do so at your own peril.
b) There will be a consensus that it is NOT cheating, and therefore you are free to do it.
c) There will be no consensus, in which case you can count on some players' doing it to gain some advantage, therefore it is de facto 'practiced cheating', therefore you do it or choose not to do it at your discretion.
In any of these three cases you know where you stand.
There's nothing like putting the issue out in the open to get a resolution, to ensure that everyone is involved in and aware of that resolution, and to influence behaviour going forward.
I acknowledge that this 'Litmus test' for PBEM cheating will not be embraced by those who want to do something (e.g. trade screenshots) to gain some secret advantage. Ultimately though, it is the viewpoints of the players you game with that matter more than anyone else's.
In general, the PBEM game allows all kinds of opportunity for rich diplomacy outside of the game's interface. Having played many PBEM games I think that interpersonal diplomacy is the ONLY redeeming aspect of PBEM, as compared with the other game modes. However, PBEM also allows all kinds of methods and opportunities for cheating; this is what makes it so important that the players agree on what is in or out of bounds.
I don't advocate any kind of cheating (in fact I have filed problem reports and corresponded with Firaxis to close certain MP cheating loopholes), however I think that you have to go into PBEM games with your eyes open. Even really good people that you trust to behave with integrity in the real world might cheat in a computer game wherein there is no way to tell that cheating has taken place, and wherein the consequences of cheating are not apparent to the cheater. To ignore this would be naive (though you may be happier come to think of it).
Sigh

- bvc