That picture is one of many free ones at pexels.com, which is an excellent resource for high-quality photos. It is my favorite from the following list. This was in the FAQ:
I see people using high-quality photos that clearly are not their own. Is it okay to use random pictures from the internet?
No, it is not allowed. You may, however, use photos from “free image” websites such as:
Pexels.com
Stockfreeimages.com
Unsplash.com
Pixabay.com
Freeimages.com