Leda2.0001_DaSesto

Cesare Da Sesto, "Leda and the Swan," ca. 1506, Wilton, Salisbury, UK, The Wilton House Collection

Zeus disguises himself as a swan to seduce the wife of Tyndareus, Leda. Sparta, where Tyndareus rules as king, is in the background. Conflating time, DaSesto shows the sinuous seducer and the gamboling offspring simultaneously present — or, perhaps more intriguing is the suggestion that Zeus returned to visit Leda (repeatedly?). The artist may invite us with this body language to see Zeus as a supportive spouse.

The offspring of this union are the Dioscuri (Castor and Polydeuces [or Pollux]), Clytemnestra, and Helen. Some versions make only Pollux and Helen Zeus' offspring of Leda; while Clytemnestra and Castor are Tyndareus' children by her, all miraculously born in one avian brood.

Leda's facial beauty in this painting is sometimes attributed to Leonardo da Vinci's own hand, though Da Sesto may have learned the skill from his teacher.
Including the swan (and all its personality) in the painting really drives into the painting's "narrative gain". This swan sticks around or returns for the arrival of the offspring. (Cf. Adam Miller's contrasting Leda.)