Pinterest was the bane of my existence when I was checking copyright on videos instructors created. They’d credit images to Pinterest and I’d have to go back to them and explain how Pinterest isn’t the copyright holder for much of anything. “But that’s where I found it!”
Oh, I know that feeling well! I actually was pretty good at tracking down rights info for randoms who would contact us with "I found this clip on YouTube and put it in my movie, now I have a distribution deal so can I license it please?"