Is it illegal? Should I contact the person and ask before I do that? How did stephanie myer come up with her book covers? Did she draw them? Or did someone else?
You definately need to contact the person first, or it is illegal. Stephenie Meyer sent her books off to her publisher who hired people to do the covers, she didn't have any say in the cover choice at all. She even has a page on her website with all the covers she designed herself that never made it onto the books.
It is illegal because it is not your property. You would need to contact the person and get the correct copyright permission. Most likely including monetary fees. I don't know about Stephanie Myer but some authors hire artists to design a cover for them.
Stephenie Meyer looked them up on google and played around with photoshop. Even though she did this, it is illegal and you shouldn't do the same. Unless you're self-publishing, then wait until you're published to get a book cover.