The custom of illustrators preparing completed paintings for consideration by publishers is an intriguing one,  for several reasons.  First, most illustrators create sizeable paintings even for a small picture on a book cover because publishers like the detail that can be created by reducing the size of a large painting to fit a cover.  Second, it is intriguing, if you think about it, that almost all such  illustrators, especially editorial illustrators, are permitted to keep the painting once it has been photographed for a book cover and retain many rights to their artwork.  You would think the publisher would keep the image they paid for, but they don't.  After a certain amount of time, the artist is free to sell their paintings and they become very collectible, especially for best-selling books.  But also because the paintings are just beautifully done.