The One With Joey's New Girlfriend - S4-E5
Continuity mistake: Chandler is chasing Kathy down the street trying to get her attention. During a close-up of him running, just before the camera cuts away we see a guy's face on the left; when it cuts back to Chandler he runs for a while and then encounters the same guy (the dog-walker). Also, when he finally lands in front of Kathy, there is a bike rider coming towards them, but as the camera cuts back and forth between the two, the rider never passes them. (00:11:30)
The One With Joey's New Girlfriend - S4-E5
Continuity mistake: When Chandler is chasing Cathy, the mustard on his shirt changes place, size and darkness throughout the scene. (00:11:45)
The One With Joey's New Girlfriend - S4-E5
Continuity mistake: During the scene where Chandler is playing hide and seek with the duck and the chick, the duck moves from facing the opposite way to him, facing the same way, and then the opposite way as Joey walks in. (00:13:35)
The One With Joey's New Girlfriend - S4-E5
Continuity mistake: In the bar, Kathy is trying to set Chandler up with one of her friends. When the camera is facing Chandler, we see Kathy's hand rubbing Joey's back, but when it cuts to a front shot of Joey and Kathy, her arm is down in front of her. Then without having had enough time to move, her elbow is up on the table and she is holding hands with Joey. (00:16:10)
The One With Joey's New Girlfriend - S4-E5
Continuity mistake: When Ross goes to Amanda's house for a date you see her two children sitting on the couch. The little boy with blonde hair is sitting down, when it cuts to Ross and Amanda talking the boy is standing up on the couch, the next shot he's sat down as Amanda walks out the door.
Answer: The painting doesn't have a name, and if it does it is never mentioned in this episode.