The skill required to create a good cover is not simply being able to use image software, but having a strong sense of whatever visual 'language' the genre of your book uses. For romance it might be attractive faces and bodies, for science fiction great space and light effects.
It's no good saying your book transcends genres, or is a western-sf-youth-romance-thriller! It may well be, but choosing the right genre to appeal to the maximum number of readers is a necessary skill, and one you will always need to work on.
That doesn't mean dozens of little overlapping scenes like a movie trailer, but one or two strong images that convey the essence of the book. An excellent resource is the archives of The Book Designer, which runs a monthly competition for ebook cover design, explaining what makes each entry a good or bad cover.
The typography is also fundamental: fonts used for thrillers are very different from those used for westerns, and they also help tell the story. A great typography resource is Typewolf, but there are many other good sites around. Here are two sites offering free fonts: FontSquirrel and DaFont.
Some channels have their own cover design software, but the results often look
amateurish. If you have design experience and can use Photoshop or the free
image program Gimp,
then you might enjoy designing a beautiful cover for your book. Canva is also another
great and inexpensive resource for book cover design.
Here are some excellent free stock photo sites: Pexels,
Flickr vintage photos.
If you don't want to do it yourself, then many of the channels provide lists of people who design covers professionally, and some are not too expensive. Even if you plan to use a professional, you must still spend serious time thinking about the book so you can suggest concepts to the designer, defining the essentials of the story and the characters and emotional mood of the work.
Colour, scale: An ebook cover must not only look good in colour but also in black and white, and shrunken down to the scale of a postage stamp. This means no red type on top of black – the red will simply disappear. The words must be strong and visible at small sizes and the image bright and clear. The orientation must usually be Portrait, not Landscape.
Metadata: The title and author data on the cover must be identical to whatever you enter in the various channel sites when uploading the book. For the author, never say 'by' or 'written by' or 'author' (author-name). Just the name itself is enough.
Colour space: the image must be saved in RGB colours, not CMYK as for printing.
Resolution: The image must usually be around 300 dpi (or pixels per inch) resolution, although some ask for only 72 dpi - check your specific channel support.
Image formats: jpg is usually preferred, and sometimes png, tif, gif or pdf.
Image dimensions: Whichever channel you choose, check their support facilities for the latest information, as recommended sizes keep changing with higher resolution devices. The classic book ratio of height to width of a cover image is 1.6, so if an image is 1600 pixels wide it then needs to be 2560 pixels high. However many sites now prefer images around 1600 x 2400 pixels, a 1.5 ratio.
The cover dimensions and image formats required by the channels are listed in the Summary Table on the Details page.
The next step provides the snippets of information you will need before using any of the channels, including ISBNs, payments and taxes. Go to 7. Details.