I know that for self-publishers, coming up with a cover image for your book may be difficult and expensive. But just a stroll through your local bookstore can give you ideas of what to do and not to do. The goal is to try to make your book not look amateur.
If you can’t afford a designer, my best advice is keep it simple. Make sure the title of the book and your name are not obstructed by the image (if there is one). Use colours well to create contrast (click here for information on the colour wheel) so that the title of your book is legible from far away. Choose an image that has meaning to the book (for free images, click here). Avoid glossy covers. And make sure your jacket copy has been edited!
So the next time you go to your local bookstore (or art gallery even!), bring a paper and pen (or your phone) and make note of the designs you like best and, more importantly, why you are drawn to them. Don’t forget that readers really do judge books by their covers.