Candid photos of people in public

I was out at the state park and photographed a man and his son having a conversation. I do not know these people and did not ask their permission. Can I legally post it on this site or any other to seek a critique? No compensation involved and no paparazzi stuff either. Just a spur of the moment candid photo that I like. The last thing I want to do is cause an issue for someone.
Brian