The One With The Proposal, part 2 - S6-E25
Continuity mistake: When Monica is in Richard's apartment she gets all worked up, so Richard gives her a hug to calm her down. She goes into the hug with her arms curled up in front of her face, but when the camera angle changes, they are at Richard's back. (00:13:50)
The One With The Proposal, part 2 - S6-E25
Continuity mistake: At the end when Chandler returns to his apartment, from the outside he opens the door with his right hand. Then it cuts to an inside view and he's opening it with his left hand on the handle. (00:17:40)
The One With The Proposal, part 2 - S6-E25
Continuity mistake: Right after Monica says, "You wanted it to be a surprise", the camera pans to Joey who shuts the door firmly, however in the next shot you can see the door gliding back open and then shutting again. (00:18:40)
Answer: The painting doesn't have a name, and if it does it is never mentioned in this episode.