We all know how obsessed with cover art I am. I really think that good cover art can help sell a book, particularly if it's a book by a relatively new author. If their cover doesn't stand out on the shelf, then they will never attract an audience.
This is why I really love the adult-oriented designs for the UK versions of the Harry Potter series. These are covers that totally allow adults who have a "But it's a children's book!" issue to get over it.
I also found these, although I am unsure where they are from or if they are actual published covers. The author's name on these makes me wonder, since I've never seen Rowling's full name written out like that before. Maybe fan art...?