The chubby boy on the back of the black bird is Ganymede. He’s being abducted by Zeus (disguised as the eagle) to serve as a cupbearer to the gods at Olympus. Above them, and upside down, the gods wait impatiently for their ambrosia.
Ganymede hovers over the staircase of the exquisite Palais Herberstein. This ceiling fresco was painted by Philipp Carl Laubmann (c. A.D. 1756) around the same time as the staircase was being rebuilt.