Most advice is to merely search in the places where "free" images are found in the first place. If you do somehow like a particular photo or image, and you find it elsewhere, you can do a reverse image search on the Web. You upload the source image to them. The results of that search will show you where else the photo is used, beginning with Stock Photo companies. If it's on such a place, you merely go to their website and purchase rights to it.
If it's not on a stock photo site, you will need to track down the artist and ask them. I have done this before. The answer may not be to your liking if it is truly taken by a pro who values their work in Benjamin Franklin notes. And then you will usually need a copyright notice. This is why stock photo subscriptions save big media companies big money on licensing... the terms are usually more reasonable.
But I have also literally hired semi-pro photographers before, paid them by the piece or hour, and they send me the photos and agree to transfer to me the copyright. That's real people, real contracts, and real photography done professionally. Not even wedding photographers like to turn over their copyrights, and so you still see "copyright" notices on wedding photos that are just annoying. But that's the photographer's right unless he transfers it.