One of the hardest parts of self-publishing is generating a good cover, either for paper books or digital ones. Places like Createspace and Lulu, among others, already have “cover wizards.”
You can certainly get a workable cover out of these “wizards,” but in my opinion it’s worth the effort to learn how to generate the artwork yourself, or at least hire it done. For one thing, there may be rights issues involved in reusing a cover created by a particular format’s “wizard” for another format. For another, you’re never going to get as much originality from a “wizard” as you can from real artwork.
Artwork for paper books, using the “one piece cover” art method, is hard to lay out. A one-piece cover is what you get by basically flattening the book out, or in the case of a book with a dust jacket, by removing the dust jacket and flattening it out. It’s easy to see that lining up the spine, end papers, and everything else can be tricky. Even changing the number of pages can affect this kind of cover design as the spine area gets larger or smaller.
Cover artwork for Kindle (or other e-books) is much simpler. It’s just a single JPG picture with art and title text. Last time I checked, preferred sizing for Kindle covers was 1563 pixels on the short side and 2500 pixels on the long side.
If you’re planning to do a lot of this kind of thing, it may be worth it to invest the time and money in learning Photoshop, InDesign, or other professional publication software. Adobe’s “Creative Suite” isn’t cheap, but it could be a good investment.