I've been wondering the same thing lately.
Here are a few examples that I've looked into...
BigStock allows it under normal license:
"e. Use Images in eBooks, including multi seat license electronic textbooks."
And this from CanStock...
"Yes! Our Standard License Agreement permits all of our images to be used on book covers, and inside books for illustration. If you think you will eventually sell over 500,000 copies of your book, you will need to purchase a Unlimited Reproductions Enhanced License."
I haven't looked into all the agencies yet, but I think this is the norm. Very, very few ebooks will ever sell 500,000 copies... and those will probably have the words "Harry Potter", "50 Shades" or "Hunger" on the cover... which means RF images will probably not be involved.