I have worked with indie cover design for only two years, but one of the most important lessons I have learned is that the awesomeness of a book cover is proportional to the awesomeness of the original working image. That said, I have collected a number of resources over the past few years and exploited them in my cover design. Here are three that I have frequently visited.
Flickr’s creative commons section was a huge find. People uploaded photographs of scenery and parts of the world that were a definite asset and made for decent fantasy covers. Some Science Fiction too. You can filter out what licenses you’d like to use. I suggest attribution, modification and commercial. Just remember to credit the original creator when you publish your book.
NASA’s website was packed with planet photos and random pictures of galaxies. Everything there was public domain and the content served well in creating science fiction covers. Public domain images are not copyrighted, usually either because the copyright expired or the original creator died at least 100 years ago. However, most of NASA’s images can’t be copyrighted because NASA is owned by the government.
Wikimedia Commons was also a very nice site to go through. Any image that is either public domain or creative commons will be in this repository and I have found the Commons to be more helpful than Wikipedia. As a rule, check the image before you decide to use it. Most will only require attribution, but some require that you don’t use it commercially, which can be bad for a paid book sale.