When Tolkien first supplied illustrations for the published Hobbit, he hoped ‘that they might serve as endpapers, frontispiece or what not’. In particular, though he did not say so at the time, he hoped that Mirkwood would be printed on the front endpaper.
It sets the scene for an adventure into dark and dangerous places in the Wild. It is also one of many expressions by Tolkien of ancient forests out of the cultural memory of Northern Europe: Nature at its most fearsome, mysterious and untamed.
(Source: myrcellas, via idril-celebrindal)