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:
How To Make A Book Cover Image Using Keynote
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.