St. Irenaeus discusses the Gnostic leader Carpocrates and his followers, and in so doing, hints at the origin of icons. He says of these heretics:
They style themselves Gnostics. They also possess images, some of them painted, and others formed from different kinds of material; while they maintain that a likeness of Christ was made by Pilate at that time when Jesus lived among them. They crown these images, and set them up along with the images of the philosophers of the world that is to say, with the images of Pythagoras, and Plato, and Aristotle, and the rest. They have also other modes of honoring these images, after the same manner of the Gentiles.
From Against Heresies, I.25.6