Images that create an illusion of depth, either as image pairs viewed one per eye or as single-image 'Magic Eye' patterns.
The story I've heard is that Scientific American did an article about Bela Julesz's random stereograms and put them on the cover.
A. Michael Noll, A. Michael Noll on 1960s Computer Art at Bell Labs
In 1965, Howard Wise put on a show of, in essence, computer art by me and a colleague, Bela Julesz, who was doing 3D stereograms, at his gallery. Some people say it was one of the earliest exhibits of digital art in the United States.
A. Michael Noll, A. Michael Noll on 1960s Computer Art at Bell Labs