The One With Rachel's Big Kiss - S7-E20
Other mistake: Matthew Perry supposedly really likes wearing a tux that turns out to have been made for Diane Keaton. Except that Diane Keaton is 5'7" and Matthew Perry is 6 feet tall, and also considerably larger than the very slim Diane. He wouldn't even be able to put it on.
Answer: The painting doesn't have a name, and if it does it is never mentioned in this episode.