JM,
They are screwed into the back of the cage in several areas.
To disguise the screws I used a caulk similar to the color of the background and then used a little sand in the caulk and it completely blends in.
I was setting the background up here. If you enlarge the picture you can see all of the screw holes. The backgrounds seem to add a little integrity to the cages.
The carpet background is very detailed with ledges that the snake can actually perch on.
It is my favorite background.