The One Where Ross Dates a Student - S6-E18
Ross: I would date her, but there is a big age difference.
Joey: Well think about it - when you're 90...
Ross: I know, she'll be 80 and it won't be such a big difference.
Joey: No, that's not what I was going to say at all. What I was going to say is, when you're 90 you'll still have the memory of what it was like to be with a 20-year-old.
Answer: The painting doesn't have a name, and if it does it is never mentioned in this episode.