Which French Rococo painter did the portrait of Jean-Jacques Rousseau shown in the picture?

Maurice Quentin de La Tour (Saint-Quentin, France, 5 September 1704 – Ibid, 17 February 1788) was a French Rococo portraitist who worked primarily with pastels. Among his most famous subjects were Voltaire, Rousseau, Louis XV and Madame de Pompadour.

Early in his youth La Tour went to Paris, where he entered the studio of the Flemish painter Jacques Spoede. He then went to Reims, Cambrai (1724), and England (c. 1725), returning to Paris to resume his studies in about 1727.

In 1737 he exhibited the first of a splendid series of 150 portraits that formed one of the glories of the Salon for the next 37 years. He was able to endow his sitters with a distinctive air of charm and intelligence, and he excelled at capturing the delicate play of facial features. In 1746 he was received into the Royal Academy of Painting and Sculpture and in 1751 was promoted to councillor. In 1750, he was made portraitist to the king in 1750, a position he held until 1773. He retired at age 80 to Saint-Quentin.

A man endowed with a lively intelligence, he founded in Saint-Quentin, his native city, a school of drawing, with travel grants to encourage young artists, and in 1783 (before his reason was clouded) he protected the Academy of Sciences, Arts and Letters of Amiens. Unfortunately for the History of art, by dying alone and mad, his exclusive method of fixing the pastel has been lost forever.