I find Michael Herring's cover painting for the Del Rey edition of Grampa in Oz very interesting. The image is not taken from any specific John R. Neill drawing, and Herring has been far more accurate in his depiction of Urtha than Neill ever was. According to the text, Urtha's "...face, hands and neck were of the tiniest white blossoms, her eyes, deep blue violets, her mouth a rosebud, and her nose and brows delicately marked with pink stems." In the close up I'm showing of Urtha's face, the white petals of the skin are visible, as well as the flower eyes.
The book goes on to describe her hair made of flowering ferns, her skirt of blossoming vines, and her waist of every flower you could think of, with pansy buttons . This is one point were Herring slightly misinterpreted the text - I think by "waist", Ruth Plumly Thompson meant shirtwaist, or blouse, not just a belt around the middle.
I also have a preliminary sketch for this painting, which differs from the finished piece. Urtha appears much less human, in a slightly different pose, and there is a much more elaborate garden background that was dropped for the final image. Grampa himself is pretty much identical to the finished cover!