John R. Neill created many dazzling illustrations for the Oz series. He was also known for hiding faces in a number of his drawings, adding another level of fun to the books. I think one of his cleverest examples is in Dorothy and the Wizard in Oz, from 1908.
In the drawing used for the tailpiece of Chapter 7, (on page 94 in the first edition), we are shown the carriage carrying the travelers through the Black Pit of the Mangaboos. When looked at closely, the right side of the pit reveals silhouette profiles of Zeb, Dorothy and the Wizard, carved out of the rock! They can be a little more difficult to distinguish in some later printings. This is just another example of Ozzy details!