One of the rarest occurrences in the world of art is when one work by a single artist has such a profound impact that it exerts an influence on that of the artist's contemporaries, sometimes for decades to come afterwards. In this century, Picasso's Guernica had such an impact. During the previous century, it was Manet's Luncheon on the Grass. Before that, discounting Michelangelo's frescos, which were really many paintings on a single theme, our eyes come to rest on Leonardo's Mona Lisa, perhaps the most influential painting of the entire millennium. Painted in 1503, the work is a mere 21"x30". It depicts in gossamer-thin layers of glazed oil and pigment, Lisa del Giocondo, the wife of a Florentine businessman, who must have been somewhat dismayed to have commissioned a portrait of his lovely wife, never to have it delivered. History doesn't record whether he ever had to pay for it, but tradition has it Leonardo kept the portrait with him the rest of his life. He died in France in 1519, which is how it came into the hands of France's King Francis I and eventually into the hands of the Louvre where it is today (albeit behind a thick layer of bullet-proof glass).
Before that though, it's doubtful there was a single major artist in Italy that hadn't seen the Mona Lisa, or at least knew it by reputation. In 1516, Raphael Sanzio painted his own ideal of feminine loveliness in the Donna Valeta (Veiled Lady). Of course, having waited 13 years to compete with the great Leonardo, the two paintings are quite different. Styles had changed. Raphael's lady is no less lovely, though not as serene, and appears a bit fussier than did Leonardo's effort. The lighting is more dramatic but the pose is identical, right down to the angle of the head, even the position of the fingers though not the placement of the hand. There is no "other worldly" smile or dreamy, extraterrestrial background as in Leonardo's, and the dress in much more fashionably rich, even extravagant, than Mona Lisa's simple black number. The hair is done up tightly under the flowing scarf of Raphael's Donna Valeta whereas Leonardo permitted Mona Lisa to "let her hair down."
The Venetian artist, Tiziano Vecellio, better known as Titian, was clearly under the influence of Leonardo's Mona Lisa when he painted Portrait of a Man with Blue-Green Eyes (a.k.a. The Young Englishman). Of course, inasmuch as this portrait is that of the opposite sex, direct comparisons are more difficult. But it's not hard to notice the similarities. Painted around 1540-45, the figure is suave, informal, and has much the same psychological directness as Leonardo's effort. And in turn, Titian's painting was also quite effective in influencing later portraitists such as Rembrandt, Anthony van Dyck, and Peter Paul Rubens.