An Anniversary Surprise - S3-E20
Trivia: Many TV stations over the years left out the original ending scene (the small party held before the Ropers leave). This scene explains how The New Landlord Mr. Furley knew Jack was supposedly gay in the first episode without anyone mentioning it. This ending reveals Roper told the new owners Jack was gay. Without seeing this ending, the storyline is unclear.
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".