Helen's Job - S2-E13
Visible crew/equipment: Janet stops Helen from leaving, says she can't handle seeing her beat. After Helen's speech about winning the war, as the camera zooms out, a piece of set equipment is briefly visible at the top of the screen. (00:16:55)
Ground Rules - S2-E1
Visible crew/equipment: Shadows of the boom and a crew person moving reflect off Jack's hair as he's talking with Janet in the kitchen.
Visible crew/equipment: A crew person's arm or equipment can be seen at the far right of the screen, when Mr Roper asks Jack for some baked beans, after the opening credits. (00:01:10)
Days of Beer and Weeds - S2-E22
Visible crew/equipment: Reflections of the monitors and other equipment are visible in Janet's glasses at the end.
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".