If you want to self-publish an eBook or paperback book you will need a cover.
For an eBook this is a vertical rectangular image, such as:
For a paperback book, you want an image with back cover text and a spine as well:
You need to make sure there is space at the bottom right for a barcode for a printed book, so that when it is bought in a retail shop they can scan it. CreateSpace will add one for you based on the ISBN you give it.
There are three ways to get a book cover for your book:
1) Purchase a custom cover from a designer. The designer will work with you on your book cover, either individually or competing for your cover (as in the case with a 99 designs book cover contest).
2) You can purchase a pre-made book cover. Designers like Damonza.com have a selection of pre-made book covers which you can order and then have your book title and name replacing the generic text of the pre-made cover. For example Damonza.com has a selection of pre-made covers here. A much cheaper service is Go On Write, though I’ve never used them myself (I’ve heard good things about them though).
3) You can create your own cover using an application like Adobe Photoshop or a service like canva.com. I don’t recommend this as there is a skill to book cover design.
When getting cover art designed or making it yourself make sure that image rights are purchased properly – you must have the rights for the images to be used (you can’t just steal an image off the internet for example).