She Loves Me, She Loves Me Not - S8-E2
Visible crew/equipment: After Jack talks to Larry about the magazine quiz Jack asks Janet if anything's been bothering her, and just as Terri walks out of the kitchen holding the two cans, a crew member's hand can be seen behind Terri placing a bag on the counter. (00:04:15)
Friends and Lovers (1) - S8-E21
Visible crew/equipment: When Janet gets ready to throw her bouquet, the boom mic dips into view at the top of the screen.
She Loves Me, She Loves Me Not - S8-E2
Visible crew/equipment: Just before Larry falls off the boat into the lake, a shadow of a boom mic is visible.
Grandma Jack - S8-E7
Visible crew/equipment: Notice the scene when Terry comes out of the kitchen. As the kitchen door begins swinging shut, a mysterious hand (which belongs to a crew member) suddenly appears from offstage to keep the door closed. (00:07:10)
Visible crew/equipment: Near the start of the show, money comes out of the ATM by error. You can see the money waiting in the slot and the crewmember's hand pushing the money out sporadically. It also comes out sideways sometimes, sometimes not and out of different parts of the slot.
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".