Revealing mistake: Any time a main house light or a lamp is switched off, it is still bright from the studio lights.
Continuity mistake: In the very first episode, there is a silver light fixture hanging from the ceiling in front of the kitchen. The light fixture appears and disappears throughout the first season.
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".