My guess: Its the episode where Moe reworks his pub to "M". The picture is when he's back at his alma mater to find answers out of his miserable life, visits a lecture by his former teacher, and is the person who answers the professors trick question about an ingredient for a cosmopolitan.

Don't know the episode title or episode/season number though.

Edit: I think it's this because it's basically the only scene I know where Moe feels confident and smart as himself.