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Well there are tools if you want to be absolutely sure a photo hasn't been just copied from another source: You can do a reverse image search with Tineye or even Google by just pasting the url (of the picture) in the search box.

Exactly. I've been using Tineye since it came out. Getty was using the professional version and had recommended Tineye to its contribuors. I intend to write about that at some point, but am just giving snippets of information at a time, because there is a lot to digest! As you probably know, you have to b careful how you use it, and not assume things.

I don't need to worry as I shoot my own photos. I also share them gratis for attribution using the Creative Commons. I also strongly advocate that, having been introduced to the idea of 1000 true fans via Cory Doctorow.