The National Gallery has a short little primer on paintings of saints and recognizing them by means of their attributes. As an example, in the painting below Saint Genevieve of Paris holds the candle which she miraculously relit. On the brooch at her neck are the alpha and omega signs. Saint Apollonia of Alexandria’s brooch shows pincers: she was tortured by having her teeth extracted.
As I delve further into the ancient history of mnemonics and mnemotechnics, I strongly suspect that attributes in paintings (like those frequently seen in depictions of Christian saints) originally stem from memory techniques that date from Simonides of Ceos (Σιμωνίδης ὁ Κεῖος; c. 556 – 468 BCE) and potentially earlier by means of the oral tradition.