Gigolo Jack or Professor Jack - S6-E4
Continuity mistake: In the scene where Terri admits to Jack that she mistook his cooking lessons for lovemaking lessons, look at Terri's bangs. They go from on her forehead to brushed back then back to on her forehead then back to being brushed back.
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".