I found this on facebook whilst procrastinating, but actually ended up working. These images come from very clever people at MIT and are great optical illusoinists. the images change their content depending on the distance away from the viewer. It works by combining high spatial frequencies from one image and low spatial frequencies of another, ( confusing i know but it works).
Here are a few to have a look at look at it very close then move a few metres away and see the image change;
Who's face is it?
astronaut or diver?
heres more on their website: http://cvcl.mit.edu/hybrid_gallery/gallery.html