As I was walking down a busy city street I saw an interesting looking man with a large ring on. I turned to snap my picture and he instinctively rolled his hand into a fist as if to pose for the picture. I simply said, "Nice ring." He said thanks and continued his journey.
You would be surprised that overwhelmingly, strangers like to have their picture made. 9 times out of 10, they don't even ask what you are going to do with their picture. In fact, the ones you would least expect to care about where you post their mug are the ones who ask, "What are you going to do with my picture?" My simple comment as I turn to walk away... "online."