This ambiguous image shows either a lady peering at herself in her mirror, or a grinning skull. To see the skull, back up a distance from the picture.
Entitled "All Is Vanity", this certainly is one of the most famous optical illusions of all time. Drawn by Charles Allen Gilbert in 1892, the illustration was not seen by the mass public until 1902 when it appeared in a magazine. It has been published many times and to this day prints of it are still sold.
The above copy of "All Is Vanity" is from an early 1900's postcard.