I wonder if you could find people or objects whose Greek names would be familiar to enough readers, and have a section with pictures and the names written out. Like
Αα, Ἀλέξανδρος → https://upload.wikimedia.org/wikipedia/commons/0/00/Alejandr...
or
Αα, Ἀθῆναι → https://en.wikipedia.org/wiki/Acropolis_of_Athens#/media/Fil... (although some readers might assume that the word illustrated is Ἀκρόπολις rather than Ἀθῆναι)
I'm not sure there are famous-enough ancient Greek people, places, or things to use for each letter, though. Optimistically, you could maybe have a few gods, a few philosophers, a very few places (on a map?), and then you might be stuck. Even using Homer brings in a challenge, because beginners won't yet know about the rough breathing mark and most would expect his name to begin with an H letter, as in Latin, rather than an O letter, as it does in Greek.
You could try to use things where a loanword from ancient Greek is still used with exactly the same meaning in most modern European languages (like περίμετρος for the outside of a circle), but this might turn out to depend a little too much on people's native languages and prior education.
A slightly weird compromise might be to use Latin transliterations alongside each word at first, like you could have the picture of Alexander and say
Ἀλέξανδρος
Alexandros
or of Athens and say
Ἀθῆναι
Athēnai
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