I don't have any Denslow artwork from The Wonderful Wizard of Oz, but I do have a piece done for another Baum/Denslow book, Father Goose, His Book. This was L. Frank Baum and W. W. Denslow's first success, published in 1899, and was a milestone in American picture books. It was the best selling children's book of 1899, and spawned a number of imitations.
The pages of Father Goose are basically a series of posters drawn by Denslow, who had already achieved recognition as a poster artist. To eliminate typesetting, and save money, the verses were hand lettered and dropped in during the printing process.
What I find interesting with the piece of artwork I own, is that the drawing looks to me as though it were drawn with the intention of printing it as it stands - the arrangement of animals, and the header of the moon (or sun) look very deliberate. Yet, in the printed book, the animals are re-arranged over 2 pages, and the moon is used for another poem altogether.
It's also interesting to note that these are the same four animals that figure in The Bremen Town Musicians by the Brothers Grimm.