Factual error: In season 4 generally, Phoebe has the shortest pregnancy ever. She gets pregnant as the boys win the girls' apartment. In the next episode Ross meets Emily, after 6 weeks he gets engaged to Emily and they then choose to get married 4 weeks later. Phoebe would only be 10-12 weeks pregnant by the time they fly to London, not in her third trimester.
Revealing mistake: Before Phoebe agrees to carry Frank and Alice's' triplets, she already has a baby bump.
Answer: The painting doesn't have a name, and if it does it is never mentioned in this episode.