Ruben's Silhouette
The b/w image above is called Ruben's silhouette - is it two people in profile or is the white a vase? Of course it's both.
Below is a series of images based upon the same optical illusion / idea - they all started off with loose red/yellow lines (see 2nd image), then each image was drawn on using black ink and later manipulated in PhotoFiltre.
Indian Faces (above).
Hippy Faces (below).
Animals
Nude
One of the original images - Vase (below).