I know I've posted this before but it happened again today and I think it bears repeating. If you're out and about and you see what looks like a group with one person taking a picture of the other members of the group in front of some landmark or pretty view, stop and say, "Do you want one all together?" They can just say no if they don't want to or if you've misread the situation (at least once I've offerred and it turned out the person taking the picture had done the same thing!) but it costs basically nothing and you can give people something they really want to have with practically no time or effort.