I have found Google image search to be an invaluable tool for finding fake posts that are using stolen or downloaded images. I want to point out one quirk that those doing their due diligence should be aware of.
When you get multiple (more than 2) hits on an image it becomes very very suspect. Probably downloaded. Follow the links and you will find the source.
When you get the nominal 2 hits that indicate the image was not found be cautious. (The two hits are usually from Google making a generic guess: "girl" or "hair" or similar) The not found result can come from either a real original image that was not downloaded and belongs to its purported owner OR from one that was just not yet found by the Google search spider. This second case is especially probable it you get one hit out of several in a group of images of the same person.
Many times recently I have hit once with an single image from a group and then explored the hit in depth by following the link and found the other images that Google had missed buried deeply in a thread. This is especially true if the one hit is on Twittter or Instagram or one of the sites that scrape from them. This usually means the entire group was downloaded despite only one being found by Google.
Just a heads up for doing your due diligence on photos using Google image search. TinEye will have the same problem.