"The Shadow of Vesuvius" by Eilís Dillon
> A Greek slave of a Roman artisan plans his escape from the city of Pompeii amid ominous signs of the approaching eruption of Mt. Vesuvius
It can be read on archive.org.
Here's how Timon (the slave) describes his idea of the painting:
> "Have you thought about how you would like to paint her?"
>
> "As Andromeda. I always think of Andromeda as being very tall and slender, with dark hair. I'd have Perseus standing by with the head of the Gorgon in his hand, just about to turn the sea-monster to stone."