The main character in this recently-released middle grade fantasy novel is obsessed with art history. In addition to memorizing art historical facts, she likes to imitate (in the privacy of her bedroom, of course) Venus in Front of the Mirror, as seen in Peter Paul Rubens's famous painting of the Goddess of Love looking pleasingly plump (c. 1613/1614; Liechtenstein Museum, Vienna). It helps that she's got the plump part going for her, too--even if it's not always pleasing outside of a Rubens painting anymore.
This book merits a special exhibition in the Middle Grade Gallery, there are so many references to major artists--from Watteau to Degas to Picasso, and lots more--as well as to specific works in it. It's also absolutely delightful. Review forthcoming, I promise.
In the meantime, there's a clue to the title of the book in this very painting: it's Cupid, otherwise known as (ahem) the small person with wings holding the mirror. For the reverse angle, compare Titian's Venus with a Mirror (c. 1555; NGA). Which do you prefer?