I don’t qualify, since I’m already one of their users, but BigStock Photo has a offer some authors might want to take up:
It’s their first offer I am referring to: “1 Image a Month Unlimited Free Trial.”
Since you’re unlikely to be writing more than one book a month, that offer could get you just the cover photo you need for one or several books out of their 15-million+ collection. Their standard license is fine for book covers and interiors.
Here’s an illustration of the types of cover images you can get from them:
I’ve found that for most subjects close-up, professionally done, full-color images of people make the best covers. And while I search several sites, I usually find what I want at Big Stock. That helps because, for most of these sites you buy pictures using credits you’ve purchased. It’s a pain to have credits scattered over several sites.
Just keep in mind that this is a holiday season sale. It may not last.