When looking for an image for either your book cover of blog posts for your website make sure it says that you can use it for commercial use. Since you are an author and writing and selling books is a business, you will be using it for commercial use.
When selecting a book cover, try to find an image that isn’t used too much and that is unique. My favorite graphic website is Pixabay. I’ve noted some other image websites that you might be interested in.
|1. The Library of Congress’ image collection|
|2. Creative Commons search on Flickr|
|3. Public Domain Images Google Search|
|4. Archive.org for public domain books|
|5. Illustrated books on Project Gutenberg|
|6. American Memory Project|