One of the most pervasive problems in the scholarship of classical paintings is that of authenticity. Traditionally, the attribution of a work of art rests on the subjective opinion of an art historian bolstered by scientific data pertaining to the types and possibly the ages of the materials of the artwork. To expand the range of technical information that may be applied to the painting authentication problem, the methods of computer image processing (IP) have been employed to compare the techniques in two paintings. One is the Mona Lisa del Gioconda by Leonardo da Vinci. The other is known as the Isleworth Mona Lisa and has also been attributed to Leonardo by a few scholars. Computer IP was used to compare statistical and geometrical features of the two paintings. It emerged that the Isleworth work is not a copy of the Louvre painting but does have numerous similarities in composition and execution. These findings lend support to the theory that the Louvre Mona Lisa may be a portrait of Costanza by Leonardo that had been thought lost.