Good question. If u are talking from a legal point of view, I wonder also if using people from the street to construct a story is allowed. 🙂 I took the photos on public domain. Still, I will add a disclaimer at the end of the post as my photos do contain people.
Still...if you want to participate you don’t have to do it as I did. It can be 4 photos of anything, not necesarly people, that tell your story.