The process of painting a picture or taking a photograph of a person is called portraiture. If an art exhibit only includes pictures of faces, you can describe its focus as portraiture.
Portraiture is the art of making a portrait, which is a close study of one person. Taking a candid snapshot of your friend riding by on her skateboard isn't portraiture. But if you ask your friend to pose with her skateboard while you take a photo — that's what portraiture is all about. Painters who specialize in portraiture often have subjects "sit" while their portraits are painted. Portraiture is rooted in the Old French portraire, "to paint."