One of the most important things you can do to successfully market your book is to create a nice image for the "cover" that will be displayed in the Amazon store.
We have found a few people to do cover image work with, but you are by no means limited to their services - you can get anyone you want to do a cover. We simply provide this as a convenience to our users.
Write us at email@example.com with the designer or designers you'd like to work with, and your ideas about your cover, and we'll go from there.
A good way to get started is to think about the visual themes in your book, and how those might make for a good cover. You can likely keep costs down by providing your own image and letting the designer retouch it and add the title and other text.
Get in touch - we'll need 4 example images you've done, at 400x600 pixels.