Today Flickr announced they have 200 million photos available on their site. Writers can explore this massive repository to find free photos to use in book covers, blog posts and book illustrations.
To use these photos, go to Flickr’s advanced search function. Scroll down, search: “Only search within Creative Commons-licensed content,” “Find content to use commercially,” “Find content to modify, adapt, or build upon.”

I haven't tried this yet so I will not vouch for its ease, but here is the info for those needing artwork for a cover- LIKE ME.