Once you've watched the movie reel you found in the ruins --
-- you'll find the wallpaper you saw in the movie on the wall behind and to the left of the spiral staircase in the burned-out building. But it's out of Nancy's reach. How to get at it?
When you look closely at the wall, Nancy says, "If I can stack enough things together, maybe I could climb up to that wallpaper," and a bunch of items appear on the screen. What you need to do here is stack those items until they reach the top of the screen and don't topple over. Read to the end of this hint for a screenshot showing one way to do this.
The things you have to work with (lined up in the top left corner of the screen) can be rotated with a right click. There are also multiples of some pieces; you can't see it, but you actually have one long board, 3 rectangular bricks, 5 square bricks, 2 hexagonal wheels, and 3 triangles.
You don't have to use all the objects, and the stack doesn't have to be one you'd actually stand on in real life. Just make a pile that reaches the ceiling without falling over, and you're good.
There's probably a way to use the wheels or triangles, but I've never found it. Just stick with the square things, and you'll do fine.
Any kind of stack that reaches the top is fine, but here's how I did it:
Behind the wallpaper is the star sketch and poem. Click on them to have Nancy make a note, then back away quickly because the roof is about to come down.