Actually, if you hire her to design the book cover for you, *you* would typically hold the rights, since this would be a "work made for hire"--a commissioned or hired work, which belongs to the person paying for it.That said, the best way to resolve this is by an agreement which makes it clear that you have the rights. If she won't sign such an agreement, that means that even though you should be the rights owners, she may be intending to reuse the art/design...and if you don't want that to happen, you should go to a different artist.