Why does he keep her photo in his pants for days??

No, I just saw the movie (for the second time) and it was days later that they pulled his pants down. Theo surprises him when he's in his underwear and kimono, and then Theo loses his bet with Isabelle and has to, um, pleasure himself in front of the two of them. Then we cut to a cafe where Matthew (fully clothed) reproaches Theo for what he did ("In front of your sister?!") Then Matthew's voice-over tells us that it was then that he realized the "stakes" of their movie games were higher than he thought. Then he tells us that "one night" he was challenged by Theo... And then we have the scene of the de-pantsing. However you look at it, there's no excuse for carrying a big, sharp-edged 60's-era snapshot against your johnson for days on end...unless he's even sicker than either of the two siblings.