What are the pictures called that are made up of a lot of smaller pictures?

A Collage, perhaps.

that's a collage (not a school)

Pheonetically: co-ladge

It is called a collage.

A picture that is composed of different, smaller pictures is called a mosaic.

A "photographic mosaic", also called a Photomosaic, is a picture that has been divided into small equally-sized sections, each of which is replaced with another small photograph.

The average color of the entire small replacement photo is more-or-less the same as the average color of the small patch of the original photo that the small photo covers.