Symbolism, Imagery, Allegory
Who likes rats? Nobody. The rats in this book look even scarier than the real thing (who can be kinda cute, btw). They've got impossibly long and skinny snouts, crooked whiskers, and tons of long, sharp teeth. They look extremely sinister, and they're shrouded in darkness. It's easy to see that although the mice and rats are both rodents, they come from very different worlds and look quite different, too.
Even within the same species, the illustrations help us to distinguish between our two main rat characters; Botticelli and Roscuro. Botticelli is huge and intimidating. In the illustration with him and Roscuro—our first sight of Roscuro—he's shrinking in horror from the light, while Roscuro stands and stares at it. When Botticelli is leading Despereaux to certain death in the dungeon, Botticelli is gleefully leering, the very image of evil.
The pictures of Roscuro change as Roscuro's personality changes. Roscuro's a pretty ugly rat, but in that first illustration, he has an awestruck expression, not a bad guy at all. In the second, his expression is shocked as the princess spies him hanging from the chandelier, watching the party. After that traumatic experience, as he becomes angry and vengeful, he also becomes uglier. His expression is more evil and sneering, like Botticelli's. The changes are subtle, but revealing.
The Princess
Our beautiful Princess Pea looks the same in all the illustrations: kind, gentle expression, radiant blonde hair. Even in the black and white drawings, you can see she glows; she's always bathed in light. In the last illustration of her, seated at the table with her loving father and mouse-knight, her sweet expression says it all:
Happily ever after.