The One With Phoebe's Uterus - S4-E11
Continuity mistake: In the scene where Joey is showing kids around the museum, there's a little boy who corrects Joey about the existence of the Mastodon. The kid's arm is by his side at the beginning of the scene. When he begins saying, "Isn't the Mastodon from the Pliocene Epoch?" his left arm is suddenly up and his thumb pointing at the Mastodon. (00:07:05)
Answer: The painting doesn't have a name, and if it does it is never mentioned in this episode.