I like the side-by-side Mona pictures here...
http://www.echalk.co.uk/amusements/OpticalIllusions/mona/mona.html
It’s fun.
While the premise is sound, and you can clearly see there is a difference in the two upside down portraits—there are also shenanigans going on as the pictures rotate. Watch closely and you will see how the eyes become MUCH more downcast and the frowning mouth more accentuated while the picture is rotating.