The One With All The Haste - S4-E19
Revealing mistake: When they are using cards to bet on the Knicks tickets, watch Joey carefully as he goes to reshuffle the pack from behind the arm of the chair. It's obvious that there are two decks, and he is in fact switching them over. (00:13:40)
Answer: The painting doesn't have a name, and if it does it is never mentioned in this episode.