Just as the book is repackaged, the cover design is a repackaging of sorts, made up of bits from the prior year's Oz title, The Road to Oz.
In The Road to Oz, John R. Neill created a pair of alternating chapter headings. One was a sort of ornamental shield, and the other was a ring of children's faces; both were used as frameworks with story-specific illustrations in the center. The ring of children became the main motif on the cover of Baum's Own Book.
This illustration has always struck me as odd within The Road to Oz, due to the manner in which it was printed. Each appearance seems soft and slightly blurred, with vertical white lines running throughout the image. It lacks the crispness of Neill's usual pen work, which is amply displayed throughout the book. Presumably, this was to create a half-tone image, but it comes across as a poorly made printing plate!
Within the ring are drawings of the Scarecrow, Tin Woodman and Cowardly Lion. These are each taken from Road illustrations, but with adjustments. The Lion is from the copyright page, with a bit of straightening and a great reduction in the amount of cross-hatching.
The Scarecrow and the Tin Woodman are taken from a full page drawing of the characters inside Ozma's palace. For the use on Baum's Own Book, the characters have been cropped and cleaned up, and the Tin Woodman has been reversed. He's also had an arm added!