Goya’s painting shows the god of time devouring one of his children, and scholars have read it as an image of old age and melancholy, although Goya left no known title or explanation. Folke Nordström saw in it Goya’s own state of mind, faced with old age and the illness he suffered in 1819. Salas and others link it to Two Old Men on the opposite wall, since Saturn is associated with old age and melancholy. Nigel Glendinning saw in it a tyrant destroying his own people.

Source: Fundación Goya en Aragón: catalogue entry 660 · Fundación Goya en Aragón: Two Old Men · J. J. Barragán, Aragón no. 393 (2022): Glendinning; title

In the myth, Saturn swallows his children as they are born because he has learned that one of his sons is destined to overthrow him. Saturn is the Roman god identified with the Greek Titan Cronus, whose story Hesiod tells in the Theogony: his wife Rhea saved the youngest, Zeus (Jupiter), by giving Cronus a stone wrapped in swaddling clothes instead. Zeus later defeated his father and made him disgorge the children he had swallowed. Goya painted the moment Saturn devours one of them.

Source: Hesiod, Theogony, lines 453–506 (tr. H. G. Evelyn-White) · Wikidata Q134388 (Saturn, same as Cronus) · Fundación Goya en Aragón: catalogue entry 660

Saturn Devouring His Son is in the Museo del Prado in Madrid, inventory number P000763, and has been there since 1881. The Prado holds all 14 Black Paintings: Baron Émile d’Erlanger donated them to the Spanish state after showing them in Paris in 1878, and a Royal Order of 20 December 1881 brought them back to Spain. The museum now catalogues the painting simply as Saturn.

Source: Wikidata Q1150997 · Wikidata Q1988253 (Black Paintings) · Fundación Goya en Aragón: Leocadia (history of the Black Paintings) · J. J. Barragán, Aragón no. 393 (2022), quoting the Prado record

Goya’s image recalls Peter Paul Rubens’s Saturn (1636–1638), also in the Prado, but the two differ: Rubens follows the god’s traditional iconography, while Goya concentrates on the cruelty of the act. Nigel Glendinning noted that Goya’s Saturn carries none of the god’s attributes, and José Gudiol saw him as an atrocious giant rather than the melancholy god of tradition. Rubens’s version is an oil on canvas of 182.5 × 87 cm; Goya painted his on a wall.

Source: Fundación Goya en Aragón: catalogue entry 660 · Wikidata Q6122115 (Rubens, Saturn, P001678) · J. J. Barragán, Aragón no. 393 (2022): Glendinning, Gudiol

Yes: Saturn Devouring His Son is Goya’s original painting, but it is no longer on its wall. Goya painted it in oil, in a mixed technique, on the plaster of the Quinta del Sordo, and between 1874 and 1878 the Prado restorer Salvador Martínez Cubells transferred it to canvas for Baron Émile d’Erlanger. Paint was lost: the canvas lost its white ground, damaged areas were repainted, and the eyes were among the parts most affected. A photograph taken by J. Laurent in 1874 shows the mural still in place.

Source: Fundación Goya en Aragón: catalogue entry 660 · Fundación Goya en Aragón: Leocadia (history of the Black Paintings) · J. J. Barragán, Aragón no. 393 (2022), quoting the Prado record

Yes. Goya died in 1828, 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 Q5432 (died 16 April 1828)