Édouard Manet painted it in 1863; the canvas is signed and dated “ED.MANET, 1863” at lower right. Claude Monet later painted a different picnic scene with the same title, dated 1866 and also in the Musée d’Orsay. According to Panorama de l’art, the art-history site of the Grand Palais, Monet’s version was both a homage and a challenge to Manet: it has no nude, and its handling of natural light announces Impressionism.

Source: POP, French Ministry of Culture: Joconde record 000PE003916 · Wikidata Q1167912 (Monet’s painting) · Panorama de l’art (GrandPalaisRmn)

Manet moved a subject from the old masters into his own time. The Musée d’Orsay describes the picture as a tribute to Europe’s artistic heritage: the theme comes from Titian’s Pastoral Concert and the central group from an engraving after Raphael. But no myth or allegory explains a nude woman among clothed men, and their modern dress made the scene look obscene to the public of 1863. The museum sees it as perhaps “the departure point for Modern Art”.

Source: Musée d’Orsay on Google Arts & Culture · Panorama de l’art (GrandPalaisRmn)

Manet’s canvas shocked with both its subject and its painting. The jury of the 1863 Salon rejected it, and Manet showed it under the title Le Bain (The Bath) at the Salon des Refusés, which Napoleon III created that year and which opened on 15 May in the Palais de l’Industrie. There it became the main attraction, drawing laughter and scandal: a naked woman among men in modern dress, painted with brutal contrasts and broad brushstrokes instead of the smooth finish of academic art.

Source: Musée d’Orsay on Google Arts & Culture · L’Histoire par l’image (GrandPalaisRmn) · Panorama de l’art (GrandPalaisRmn)

It is classed as Realism: Wikidata gives its movement as French Realism. It is not an Impressionist picture, but it opened the way for later painters. Panorama de l’art notes that its questions about modern subjects, perspective and colour contrasts gave Manet’s successors a vast field to explore: Claude Monet answered it with a version in natural light, Paul Cézanne drew on it in the 1870s and Pablo Picasso painted a series after it nearly a century later.

Source: Wikidata Q152509 (movement) · Panorama de l’art (GrandPalaisRmn)

Luncheon on the Grass is in the Musée d’Orsay in Paris, inventory number RF 1668; the canvas measures 207 × 265 cm. Manet kept it until 1878, when the baritone Jean-Baptiste Faure acquired it; it went to the Durand-Ruel gallery in 1898 and then to the collector Étienne Moreau-Nélaton, who gave it to the French State in 1906. It entered the Louvre’s department of paintings and was assigned to the Musée d’Orsay in 1986.

Source: Musée d’Orsay record (RF 1668) · POP, French Ministry of Culture: Joconde record 000PE003916 · Wikidata Q152509 (owners, collection)

Yes. Manet died in 1883, so his paintings are free of copyright everywhere. A faithful photograph of a public-domain painting adds no new copyright in the United States or the European Union, so you can download this image and use it in personal or commercial projects. Public Domain Images for Commercial Use explains how.

Source: Wikidata Q40599 (died 30 April 1883)