This is from a mosaic with the Zeus carrying off Europa.
From the Enc. Britt.: Europa [is] in Greek mythology, the daughter either of Phoenix or of Agenor, king of Phoenicia. The beauty of Europa inspired the love of Zeus, who approached her in the form of a white bull and carried her away from Phoenicia to Crete. There she bore Zeus three sons: King Minos of Crete, King Rhadamanthus of the Cyclades Islands, and, according to some legends, Prince Sarpedon of Lycia. She later married the king of Crete, who adopted her sons, and she was worshiped under the name of Hellotis in Crete, where the festival Hellotia was held in her honour.
A knowledgeable viewer informed me: this is perhaps Astypalaia (Europa's sister who was abducted by Poseidon). She's riding a sea-leopard (pardalokampos).