From the Super Mario Wiki
The Gallery is an area in Luigi's Mansion located in the vicinity of Professor Elvin Gadd's laboratory. In here, one can see the pictures of Luigi's captured Portrait Ghosts. Each of its two halls leads to a room with a large portrait of King Boo. On either side of his painting are two angel statues, which will flap their wings if they are interacted with. A picture of Luigi's NEW Mansion also appears after beating the game.