The one on the left is, of course, by Leonardo and is a study for his Madonna of the Rocks. There are some quite interesting links on Leonardo's work, including one that argues the Mona Lisa is a self-portrait of the artist; also, a quite interesting theory about her smile, and a final one about her identity. Here is a place for more information about the Madonna of the Rocks (Leonardo's model is quite lovely. Some might have argued that she could have been the model for the Mona Lisa.)
