This is one of the stranger optial illusions I've seen a quite a while. It's easy.
1. Stare at the dots on the girls nose for 30 seconds. (Took only about 10 ro 15 for me).
2. Look at any white area of your screen. Such as a white portion of your browser window.
When you blink you will see an image of the girl. But it won't be in negitive. It's in full color.