Unfortunately, the image at the top of this page is not a real Denslow drawing. William Wallace Denslow illustrated The Wonderful Wizard of Oz and created the visuals for the memorable characters created by L. Frank Baum. When the book was first published in 1900, Denslow's illustrations received as much praise as the story itself. Much of the original artwork for the book is in the collection of the New York Public Library, although there are lucky collectors out there who have original pieces from the book.
The closest I've come was an illustration offered on eBay years ago, created by Denslow for the Father Goose Song Folios, from 1900. An image of Dorothy and Toto was part of this drawing, which unfortunately I didn't win. The drawing was created for a poster, which was reprinted on the rear cover of the 1968 Baum Bugle, published by The International Wizard of Oz Club, also known as IWOC.
The piece I'm showing at the top of my blog was created by Dick Martin for the 1964 Reilly & Lee edition of The Wizard of Oz. It is a re-drawing of the endpaper design created by Denslow for the 1903 Bobbs Merrill edition which was called The New Wizard of Oz. (The bibliographic details of the Oz books are quite complicated, but there is an excellent guide available called Bibliographia Oziana, sold by IWOC). Dick Martin did a lot of Oz illustration, and really captured the look of the original Denslow piece.
Chances are, this is as close as I'll get to an original Wonderful Wizard drawing, although there are other Denslow drawings out there with the Scarecrow, Tin Woodman and Cowardly Lion - most notably, the comic page he created in 1904-1905. Drawings do surface from this, so there's always a chance!
On the left, the original endpapers - sorry about the glare!