Stock photos are sold by size. The larger the size, the more costly they are. The photos are sold by “credits” and a credit seems to equal $1.36.
For an ebook cover, I’ve learned that a large image on the cover needs to be medium size, otherwise it looks pixelated.
You might want to search for your own images for your cover (it’s fun).
The stock photo site offers a free “comp image” that is high quality but has a watermark on it, so that you can see how it’ll really look on your cover before you decide to buy it.
The stock sites have folders called “lightboxes” where you can place images you’re considering. That makes it easy to exchange information about stock photo images. They are all numbered with an id number.
For more info, this site is excellent: All About Stock Images