Why do people put up random pictures on their profile?

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Why is it that I see so many pictures on here (on peoples profiles) that obviously weren't taken by them? Do they steal these images from the internet? Where do they find them and why do they use them? It is one thing to take a picture of something and post it because you took it or to post pics of yourself or family and friends but why post pics of people you have never met. Or places you have never been, with no comment as to why you have posted it? I can understand if someone has a pic of a place and says something like "I want to live here" or "I wish I could be there right now"...But they give NO explanation. The pics of people they have never met is the one that really bothers me, especially when they post pics of someone elses children. That is weird to me. And kind of creepy. I also could understand someone posting a picture of a place and discussing it in a topic. Everything else befuddles me. Someone please explain the benefit of posting pictures that don't belong to you and are of no use...