The One With Joey's New Brain - S7-E15
Character mistake: After Susan Sarandon slaps Joey the first time, after he tells her that her character is dying on DOOL, she turns away and right before she is off camera she smiles, instead of maintaining her anger. (00:06:05)
The One With Chandler's Dad - S7-E22
Character mistake: When Chandler says "and there's daddy", an extra behind him chuckles in time with the studio audience. He's the only one laughing and it happens right on cue with the audience.
The One With Phoebe's Cookies - S7-E3
Character mistake: When Phoebe brings in her frozen cookie and Monica tastes it to try to determine the ingredients, she says "I definitely taste nutmeg." She then goes on to brag about how that's the difference between a professional and a layman. Except that later when she finds the actual recipe, it's from the Nestle Toll House bag - and that contains no nutmeg.
Answer: The painting doesn't have a name, and if it does it is never mentioned in this episode.