Just a placeholder post with links to sites that have free, public domain, Royalty Free pictures which can be used for doing book covers. ALWAYS read the fine print and make sure, before you try to download, what the rights are so there aren’t any nasty surprises later. I download into folders named after the site the pics are gotten from with a copy of the rights and Terms and whatnot so when I do go back to use the pics I’ll know where they came from and what I am working with. A little bit of work now will avoid confusion later down the road.
http://hubblesite.org/gallery/album/ <——- omg the pics here are gorgeous!