Why do people post pictures of others?

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I often see on sites whether it is myLot or a website or a social networking site that people post photos of not just themselves but ones of friends or family. Sometimes it is a picture of them, but maybe friends or family are in it. Othertimes it is just other people and not even the actual person in it. I am confused at this. Do they get permission to post these peoples picture? I would be very upset if someone I knew posted a picture of me on a site, even if it was just a photo I was in with them. The internet is worldwide, permanent and people can easily take/steal your property and use as they wish even if it is on your own site or profile. Personally if I wanted any form of a picture of myself I would post it myself. Things are not the same as back when you develop a photo and the most that anyone who saw it was the film developers or a few strangers. Posted photos mowadays are available to the infinite and endless world. One time I was on a board and a lady said that her avatar was a picture of her daughter. I thought that was horrible. She was upset because she was getting responses from young guys. What would you expect, her avatar was of a teenage girl. Your avatar is how people identify you. I would be mad if me mom used my face as her avatar. Anyway with all my ranting... what do you think? Do you put other people photos on your networking site/websites or as your avatar? Do you tell/ask the people that you are publicly posting picture of/with them? Thanks for taking the time to read/respond.