Continuity mistake: When Ross introduces himself to Mona, the reception place card that Mona's holding switches from her left hand to her right hand, between shots.
Revealing mistake: In the one after Monica and Chandler get married, Chandler says he can't dance because his shoes are slippery. If you look closely, he managed to slip so much by standing on the bottom of Monica's dress, not because of the shoes. (00:06:00)
Continuity mistake: Just as Ross and Mona are about to dance, her arms are crossed. The camera cuts and they are on her hips. (00:16:05)
Continuity mistake: When Monica and Rachel are discussing Phoebe, Rachel's hands go from playing with the table to dry washing depending on the shot. (00:12:30)
Answer: The painting doesn't have a name, and if it does it is never mentioned in this episode.