This is a simple hybrid image based on the work of Aude Oliva, Antonio Torralba and Philippe Schyns, http://cvcl.mit.edu/hybridimage.htm. When you look at the large image up close, Mona appears to be smiling; however, if you stand back at a distance (or look at the thumbnail), she appears to be frowning.

The hybrid was created by making two crudely modified Mona Lisas: one smudged into a smile and the other into a frown. Then the smiling Mona is high-pass filtered while the frowning Mona is low-pass filtered. Since human visual perception is a band-pass phenomenon, we tend to see the high-passed image better when we're close up, while the blurred, low-passed image is more obvious from a distance.

The possibly amazing thing is that da Vinci's original Mona Lisa painting displays a similar phenomenon — if you low-pass filter it (or blur your eyes or stand at a distance), she appears to be smiling, while if you high-pass filter it, she is not smiling. Perhaps this is part of the enigmatic quality associated with her smile. (See http://cvcl.mit.edu/hybrid/Talk_Hybrid_Siggraph06.pdf, which includes the filtered Mona Lisas and many amazing hybrid images.)

 

One of da Vinci's early failed experiments

See also: Masterpiece Helper, http://www.museumofconceptualart.com/masterpiecehelper.