Why do so many upload pictures where they don't have the copyrights for?

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I find it strange that they pay here on my lot for uploading pictures, when it's obvious that many of those pictures aren't uploaded by the ones who have the rights to it. Not that i really care, cause all those copyrights doesn't really matter to me. It's just that I personnaly couldn't live with the thought of being payed for someone elses work. What's the purpose of all those pictures anyway? If they accompany a certain discussion to make things clearer i can understand. But often those pictures are just there, like this is some kind of picture library...