There are no requirements. But the truth of the matter is one of the most important things for the business of writing articles on the web is the pages need to be attractive, and so you tend to need some attractive and highly relevant photos.
These days they want you to use photos that are full width, but that isn't a rule either.
I try to have a photo after ever three to five hundred words, if possible. Don't listen to the people who act like you can't use this or that photo. Just credit the photo by linking to the page you got it from and if the photographer's name is available, certainly credit them.