The One Where Chandler Takes a Bath - S8-E13
Other mistake: In the opening scene, while Joey's eating the bowl of Frosted Flakes the box is on the counter, and the brand name "Kellogg's" has been painted blue with its white line extended and covered with a sticker. At the end of the episode, when Joey's eating the Cocoa Puffs the box on the counter has the brand logos for General Mills covered with stickers on the front and sides.
Answer: The painting doesn't have a name, and if it does it is never mentioned in this episode.