Originally Posted by ilovejedd
I believe for artwork and the like, "Creative Commons" is usually the license used for freely distributable content. ...
True, but "freely distributable" doesn't mean that they can be altered. Using them for a book cover would be altering the image. Only when the CC work doesn't restrict changes and alterations (the No Derivatives flag) should you use it as a component in a cover. Also, CC works often require that give the creator credit for the work (the Attribution flag in the CC license).