The Spinning Dancer is a popular optical illusion that even has its own wikipedia entry.
The illusion is the following:
The illusion was created by web designer Nobuyuki Kayahara and the question is “Which direction is the dancer spinning?” That is, is the dancer is spinning clockwise or counter-clockwise?
“If the foot touching the ground is perceived to be the left foot, the dancer appears to be spinning clockwise (if seen from above); if it is taken to be the right foot, then she appears to be spinning counterclockwise.”
A nice graphic that illustrates how the dancer can be observed as spinning in either direction is below. By simplying adding some lines to the original image you you can give direction to the dancer. The left image shows the dancer spinning clockwise, while the right image shows the dancer spinning counter-clockwise.