Plot, picture and thoughts

Picture

The sentence reads: Dikaiopolis, making a libation, prays Zeus to keep them all safe.
ὁ Δικαιόπολις looks like a real priest in front of his altar πρὸ τοῦ βωμοῦ  What is he doing? Offering prayers and libation (wine, milk, drink of honey...) to the father of Greek gods, Zeus, so that he keep them all safe on their journey to Athens.
ἡ σπονδή, τῆς σπονδῆς  is sacrifice, libation, usually wine poured on the altar while the god's help/favour is invoked.
The altar stands in the courtyard (which we can't see but have to accept as such)
ἔστιν βωμὸς ἐν τῇ αὐλῇ.
ὁρῶμεν τὸν Δικαιόπολιν ἐν τῇ αὐλῇ 
      σπονδὴν ποιούμενον 
      εὐχόμενον 
      τῷ Διὶ εὐχόμενον 
      εὐχόμενον τὸν Δία σῴζειν πάντας 
We pray to the gods (dative), but we implore them (accusative) to do something.
As for σῴζω it means both Ι rescue and Ι keep safe.

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