The One Where Ross Finds Out - S2-E7
Other mistake: When Rachel leave her message on Ross' answering machine she mentioned the name Michael after 17 seconds and "I'm over you" after 25 seconds. When Ross listen to the tape he asks "Who is Michael ?" after 8 seconds. Rachel flips the phone out of his hand after 18 seconds so he couldn't have heard her "I'm over you." (00:10:35 - 00:12:50)
Answer: The painting doesn't have a name, and if it does it is never mentioned in this episode.