The man in the top right corner is Philip II of Macedon. According to legend, Zeus took to the serpent form and seduced and had coitus with his wife Olimpias and fathered Alexander the Great (or Alessandro Magno in Italian).
This is why Zeus is shown in the half serpent form. The legend also says that Philip caught a glimpse of this and hence one day would lose the eye through which he saw this. That is why Romano shows him being blinded by Zeus's eagle.
Philip lost his eye in the siege of Methone in 355-354 B.C. a year after Alexander was born.
Plutarch's The Life of Alexander details the legend. I quote the relevant sections:
Once Philip found his wife asleep, with a large tame snake stretched
beside her; and this, it is said, quite put an end to his passion for
her, and made him avoid her society...
We are told that Philip after this portent sent Chairon of Megalopolis
to Delphi, to consult the god there, and that he delivered an oracular
response bidding him sacrifice to Zeus Ammon, and to pay especial
reverence to that god: warning him, moreover, that he would some day
lose the sight of that eye with which, through the chink of the
half-opened door, he had seen the god consorting with his wife in the
form of a serpent.