I think a lot of people do use pictures without permission and get away with it, but It's definitely a higher risk doing that than using Creative Commons images with the correct licence, like Unsplash.

Medium's rules when you sign up say "we require users to have permission to post the content they publish on Medium" - but no, it doesn't spell out that you must have permission to use photos explicitly, for those who are ignorant of such matters.

There have been a number of articles around on this topic, but overall, if you don't have your own photos, then Unsplash is a good resource - with a direct link to Medium through the magnifying glass on the plus sign.