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.
Visible crew/equipment: When Jack opens the medicine cabinet to get the shaving cream, in the mirror's reflection you can see the camera as well as where the set ends.
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: In the episode "A Camping We Will Go", when Lana and Mr. Furley show up to the Cabin unexpectedly, watch closely when Lana tries to sit next to Jack on the couch.if you look through the window by the flag hanging on the wall you'll see Larry's girl, Laura, waiting outside by the front door (in her bright pink outfit) waiting to make her entrance.
Double Date - S3-E1
Visible crew/equipment: At the start, while Mr. Roper is in the kitchen the faucet sprays water and the doorbell rings. Note the cabinets and wall behind him. When Roper walks out of the kitchen, as the swing door is open we can see the kitchen, and the upper cabinets are split apart with the wall missing, so the area behind the kitchen set is visible. (00:00:50)
Visible crew/equipment: When Mr. Roper gets the idea that it's Jack's diary, Jack tells him not to worry and it'll be their own little secret, then in the next wide shot we can see the bank of set lighting at the bottom of the screen. (00:21:15)
An Anniversary Surprise - S3-E20
Visible crew/equipment: At the end of the episode when Jack falls back off the chair, a boom mic is very visible above him. (00:25:15)
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.
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)
She Loves Me, She Loves Me Not - S8-E2
Visible crew/equipment: When Larry hops out of the canoe over the magazine panic, saying, "I gotta see Jack," the boom is quite visible at the left as the camera pulls back.
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)
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: A large shadow of a crewman's shoulder is visible on the wall behind Larry after Janet exits to the kitchen. (00:16:37)
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.
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".