In this picture, the primordial god of the upper air, Aether, is depicted wrestling a lion-headed giant.
Is there a myth/story for this anywhere?
Herakles did not wear the skin of the Nemean lion, but that of a certain Leon (Lion), one of the Gigantes (Giants) killed by Herakles whom he had challenged to single combat.
Source: Ptolemy Hephaestion, New History Book 5 (summary from Photius, Myriobiblon 190) (trans. Pearse) (Greek mythographer C1st to C2nd A.D.)
Unfortunately, I do not think a tale of Leon battling Aether survive.