Trivia (CXXV) - The legal question

Incoming:

Hi Ricardo 

I have seen your great pictures on Ello and your Blog. They are so special and interesting. I travel a lot and take pics but I'am never sure about the legal part of displaying them. Making pics of building or landscape is quite boring. People are what makes the difference. 

How do you solve the legal problem? If you make photos of strangers how do you get their approval and authorization for displaying them on the web or publishing their photos?

(...) 

Thank you in advance
Best regards

H.H.