Screenshot requirements

I'am a bit new here so maybe this sounds as a very stupid question, but what are the exact screenshot conditions for a game? I've uploaded my first one yesterday but I'am getting the message :

Rejected: Quest conditions have not been met. Please check carefully the condition of the task or upload a better quality screen.

Fine, but I don't see anykind of condition anywhere in the quest menu except : 'Play 5 battles'. It doesn't say when I have to take a screenshot. Before the game starts, during the game, when it's completed or something else. Is there somekind of list on the site with better instructions concerning screenshot conditions, so I can do it right from now on and actually earn bananas?