Trivia: In the episode where Janet asks Jack if she's a bad kisser, take a look at Janet when he kisses her. Jack unexpectedly slip Janet his tongue (which surprises the audience as well) and that throws Janet off and shocks her. Her surprised face is not acting, and was really surprised he did that. You'll also notice that when they part, she can't keep a straight face.
Trivia: Of all 5 roommates (Jack, Janet, Chrissy, Cindy, Terri), the only parent of a roommate never to appear in any episode is Cindy's mother.
Trivia: Paula Shaw plays Joyce DeWitt's mother in this episode, even though in real life she's only eight years her senior.
Trivia: Jeffrey Tambor, Anne Schedeen, and Jordan Charney have all guest starred in episodes involving Chrissy, Cindy, and Terri. (To avoid confusion, Tambor's episode with Chrissy was technically an episode of "The Ropers" but in recent years played in syndication as "Three's Company").
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".