I find it odd that he was very inconsistent in his choice of materials and sizes for these pieces. Most of the Baum covers are painted on a rigid board similar to foamcore. Other pieces are painted on masonite, stretched canvas, canvas board, and stretched linen. The sizes range from this piece at 14.5" x 24.5" to pieces that are 24" x 36". I would have thought it simpler to have one size and one material to work with for the entire series, but that's just me. I suppose, since they were not all being done at one time, he worked with what he had on hand, or in the method he was currently comfortable with. The variety does add another level of interest to the pieces.
This is a straightforward adaptation of Neill's drawing without any major changes. On the painting, the Oz banner has been dropped a bit lower than in the drawing. When viewing the painting in person, you can see that the banner was originally painted higher, then painted out and lowered. This was done so that it would not intefere with the Oz in the book title. The jagged rocks have been added to the foreground, and due to the proportions of the painting, the composition is not as tight as that of the drawing.