"Most people are quite surprised by the pictures when they see them right side up. The illusion illustrates the fact that right side-up and upside-down faces are processed differently, which some people take as evidence that parts of our brain are specialized for (normal, right side-up) face perception."
There is another one:
A Distance-Size Illusion
Now look at the image given below & can you make out who is taller of these two?
"Most people think the woman on the left looks considerably taller, yet if you click on the image you'll see that parallel lines can connect the tops and bottoms of the two figures, revealing that they're exactly the same size. This illusion illustrates the fact that the actual size an image projects onto your retina isn't the only thing that affects perceived size. The other crucial factor is distance. Objects that are farther away project smaller images onto your retina, so when you know something is far away, your visual system compensates to create a sense of perceived size that tends to correspond more closely to the actual size. In this case, the background of the path through Harvard Yard provides depth cues about where the two people are with respect to each other. Since the woman to the left looks farther away, she looks bigger even though the two figures are in fact the same size in the 2-D image.
There are many versions of this illusion, but a particularly compelling one comes from Roger Shepard. It's hard to believe that the two below are the same size, but measure them and you'll see..."
Illusions Of Faces
Design Interior: Optical Illusions In Reality
Aug 14, 2007
They may look a bit different, but we don't think they should seem too odd & in the second image one can see the faces right side up & the difference.