Depending on how you view this picture, you can see either two children holding a dog, or a skull. If you can't immediately see the image of a skull, back up a few feet.
The full title of this ambiguous image is "Life And Death As Seen Far And Near". It appeared in a magazine in 1886.
This is just one of many illustrations that have been made where objects within the picture form a skull when the picture is viewed from a distance. The most famous of these is "All Is Vanity" by Charles Allan Gilbert.