Plot hole: How could a blind man (Jim Walsh) just find Jack's apartment like that? Phoning from the Reagle Beagle, Chrissy simply states "you're nearby to us" and then hangs up the phone (without giving the address). Even if he was given the address, being blind he would not be able to find the building, let alone the right unit that quickly, but he knocks on the door within 5 min of the call.
Revealing mistake: The lights around the stage reflect in Jim Walsh's sunglasses a few times.
Answer: I believe you are referring to the artwork of Alphonse Mucha. On the wall next to the front door was "Feather" (which is one half of "Primrose and Feather"). Above the telephone in early episodes was "Zodiac". Also seen through The Apartment was "Spring" and "Autumn".