Visible crew/equipment: When Janet's father hands Jack the big gift, a microphone appears for a second at the top of the screen. (00:03:15)
Visible crew/equipment: In the scene where Maxine comes out of the bathroom and yells "Jack" a microphone appears for a second at the top of the screen. (00:25:05)
Upstairs Downstairs Downstairs - S5-E1
Visible crew/equipment: Shadows from the booms are quite visible in varied takes, along with false ceiling in the living area of their apartment.
Visible crew/equipment: Obvious shadows from booms, overhead, and crew members reflect off the walls while Jack and Larry are talking, and Larry looks like he's either reading from cue cards or following an off-camera director for his blocking moves.
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".