Collage is considered fair use, and therefore not subject to copyright law.
The doctrine of fair use protects educational and scholarly purposes such as news reporting, literary criticism, and libraries. Artistic uses are not explicitly protected by fair use, and commercial uses are explicitly not protected.
My collage can freely use copyrighted material, as long as I use no more than 5% / 10% / a small amount of the original work.
The doctrine of "de minimis" theoretically protects minimal copying, but it's vague and difficult to pin down.