If you're a self-published author and you have the resources to do so, you should definitely think of paying a professional designer to create a book cover image for you!

However, if you don't have the resources to do that right now, or if you're just getting started on a new book, you can create a good-enough (or sometimes great!) cover image yourself, using a variety of apps and online services.

If you want to make your own book cover image using familiar tools, you may find the following Leanpub articles helpful:

http://help.leanpub.com/en/articles/5340489-how-to-make-a-book-cover-image-using-keynote

http://help.leanpub.com/en/articles/5343361-how-to-make-a-book-cover-image-using-powerpoint

Please note you can precise guidance for book cover image sizing on the Book Cover page for your book here:

...making sure to replace YOUR_BOOK with your book's unique web address.