The Case of the Missing Blonde - S5-E21
Plot hole: After being stuck inside the phone booth, the police officer arrests Larry for vandalism after Larry breaks the glass in the phone booth. In a real-world situation, an officer would most likely let someone go without arrest if the person got stuck inside like that (declaring an emergency) and couldn't get out without breaking the glass.
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".