I have mentioned before that I submit my images to an agency for image licensing. The stock agency that I work with is called Arcangel. I have been licensing images through them for a few years, and have had a great experience working with them. If you are a photographer that is interested in becoming a contributor please check out their website.
Arcangel Images
Here are a few of the images I have had licensed for book covers. Sometimes they use the image as is, but sometimes they combine it with other images or edit it to suit their needs. I have no say over this process so if you are a photographer that doesn't want your work tampered with than this might not be for you.
Here are a couple of examples where the images are pretty much left as is.
Here are some examples of how an image might be altered.
Sometimes the image is so different that I don't even recognize it as my image . 
Occasionally I have had the same image licensed for different covers.
I find this particular example amusing because of the originals. Here are the behind the scenes images.
That's me... and that's also me
When I started a few years ago, I had a million questions and didn't know anyone who had licensed images before. I had to learn as I went what the process was like and what to expect. If this is something you are interested in please do not hesitate to ask me about it. I would love to answer any questions you have!