The Mona Lisa is a 16th century Earth painting, oil on canvas, created by Leonardo da Vinci. The paint is perhaps most famous for its simplicity, displaying a common human female and not anyone famous or religious, and for its enigmatic meaning. During the 20th and 21st century, the painting was housed in the Louvre, in Paris, France.
Montgomery Scott classified Guinan's facial features as being similar to the Mona Lisa, in that they both had enigmatic smiles. (Star Trek novel: Engines of Destiny)
Commodore David Fein claimed he didn't see what the big mystery was about the smile, since a person getting their picture taken was supposed to smile. (TOS novel: Ex Machina)