According to my research, the rules vary according to country, state, county, & city where the photo is taken. I know when I was working in the hospital — no patients (or staff) could be photographed without their WRITTEN consent. Even surgical consents clarified that any photos taken during surgery would only be used for educational purposes — not commercially. That was hospital policy and we could be fired for violating it. HIPPA — etc. So beyond the ethical issues of using photos of strangers for one’s own personal gain — there may well be laws in place in the municipality where the photo was taken that prohibit the use of that photo without consent of the subject. A random photo with no location indicated is definitely a no-no per Quora.

I hope this helps you understand where I’m coming from with all this. :)