My vote for the most boring and tedious type of optical illusion is the painting or drawing in which smaller figures are hidden in larger figures. Yet one of my favorite paintings is an amazing example of just such a technique. It was in the late 1960s and I was in New York for an American Physical Society meeting when I visited the Museum of Modern Art and was absolutely transfixed by the incredible painting shown above, "Hide and Seek" by Pavel Tchelitchew, completed during the period 1940--42. I spent almost all my time at the Museum appreciating it. Alas, I have not found a very good image to post here, but you get the idea.

To end, about another Tchelitchew work, "Adam's Grave"?

Pavel Tchelitchew (1898--1957)