As you know, your book will be judged by its cover. You definitely want something evocative and beautifully composed, a design that communicates the quality of the words inside. To find an excellent image that won’t set you back hundreds of dollars, visit Foter.
It’s a pleasant way to find and download Creative Commons-licensed photos on Flickr. They’re all free to use–no payment is required. What is required is observation of each photo’s particular Creative Commons license. Some CC license variants allow you to modify the images, and some don’t, for example. But one thing they all require is attribution. Foter’s automatically generated attribution embed code (example below) makes this easy. For the book itself, make sure to include the attribution shown in text form in a credits page, ideally with a link.