Book Covers

I'm naive in this area. For Natalie's Story I just uploaded a picture of the beach from when we went on vacation a few years ago. There is a beach scene in Natalie's Story so it's not a huge stretch, but still not the picture I'd like to use for the cover.

I've read about websites where you can buy a photo and use that for your book, but I have no idea how these things work. I need simple instructions in English, not lawyer talk, to follow.

Can anyone clue me in?