I’m not suggesting this is better than purchasing a more professional oriented photo from one of the online photo sites. Most of the larger, pay-for-image sites post the best quality resolution, creative ideas and offer greater options for image searches. And you can be sure that the photographer posting the image owns the rights.
flickr is for that photo that someone might have taken on vacation, that special image that is just what you are looking for. I did a search for ‘doors’ and seven pages with 42 images each showed up for me to browse.
When you go looking for a photograph for your cover be sure of the quality. That the image is scalable to the correct dpi/resolution. That you do have the rights to use it on your book cover or sales materials. If you need to use the photographers name or not. Check the creative commons license to be sure. And most of all that the photographer that posted the image actually owns the rights to use it this way.
Options are growing all the time… this might work for you. Here are some related links you might be interested in.