In a NYT review of some illustrated fairytales, Maria Tatar says the following:
Though Zwerger’s watercolors are sometimes disturbing, the decorative beauty of her work also functions as an antidote to the violent content of the tales. This dynamic is reversed in Hague’s “Read-to-Me Book of Fairy Tales”: Allison Grace MacDonald’s gentle prose mitigates the ferocity of some of Hague’s illustrations.
In other words, a beautiful picture can moderate violent images in a horrific story. Likewise, a sweet, innocent story can be spiced up by ferocious and daring illustrations.
Sometimes publishers commission artwork and realise the art too closely matches the grotesque story. A good example are the illustrations by Rosemary Fawcett which originally accompanied Roald Dahl’s book of poetry, Dirty Beasts. The illustrations are about as freaky as the poems, soon replaced by the looser, sketchier, sunnier illustrations by Quentin Blake which have since become iconic.