Chris the Story Reading Ape posted this link; it’s important reading. For the record, the pictures I’ve begun using are from stock photo sites that state expressly that they can be used for blogs, book covers, web sites, etc. These sites use the terms “resale and distribution” in ways that I find confusing; for example, depositphotos, which I have used quite a bit, says that the regular license is fine for book covers, but then says this license is only appropriate for ebooks if the photo file plays “a minor role” in the product. Since many of the book-cover designers on Mark’s List at Smashwords use sites like this, it would seem to this non-lawyer that the images are available for such use in ebooks just as in regular books.
In any case, I have found the sites I’ve used to be easy and inexpensive. You can buy images for as little as $1.00. The article this post links to lists a number of free sites, including Creative Commons sites. When I checked out Wiki Commons, I wasn’t particularly impressed with the selection there; the pay (royalty-free) sites seemed to have a larger selection of what I was looking for.
Perhaps others can share their experiences and expertise on this topic!