Can’t remember where you found an image? Want to find out where the image you want to use originally came from and who holds the copyright? Use TinEye, a reverse image search.
There are 2 ways to search: Upload an image from your computer or copy-and-paste the URL of the page or image you want to check.
This video is a bit old — made when TinEye was still in beta — but it’s a good introduction to how the site works: