Windows appear regularly throughout the game, Ib. The ones in the main gallery give a view of outside, while those inside the cursed gallery are connected to an alternate room.
Sightings
The first two windows are located in the lobby of the Guertena Art Gallery, next to the reception desk. Ib can look through to confirm it "nice and bright outside." Another window is located on the upper floor, just after the stairs. After the lights dim inside the gallery, Ib cannot see anything out the windows. If she examines the second window in the lobby, blood-like paint will ooze. If she walks by the upstairs window, a person's shadow will walk by. When Ib examines it, that person will bang on the window, cracking it.
In the Red Area, a window will be on the wall of the small room where The Lady in Blue is plucking petals off Garry's rose. When she spots and chases Ib, and Ib runs outside, the Lady in Blue will climb onto a stool in her room to break through the window, continuing to chase Ib.
In the Gray Area, a window will be in the Colored Ladies' room, outside the small room where Couple hangs. A frog's shadow will hop across when Ib walks by. Another window is also inside that room, and the Lady in Blue will use it to break inside. This can be prevented if Garry blocks the window with the bookcase.
In the Brown Area, two windows will be in the hallway where the light fades in and out. A shadowed man will walk by both windows, and bang on the right one if Ib examines it.
In the ending A Painting's Demise, the windows in the gallery's lobby will become red as the gallery gets darker.