One website I've been using to trace images has been TinEye.com. From it's website, "TinEye is a reverse image search engine. It finds out where an image came from, how it is being used, if modified versions of the image exist, or if there is a higher resolution version."
Just this evening, I looked back at an old post I had written, with a picture of a half empty glass. I looked up that image with TinEye, and saw that this was actually not for free public use. That picture was deleted instantly! A public domain image replaced it quickly. My blinders are off, and I'm careful about what I'm doing, more and more each day.
Even images that are reportedly public domain are traced by me on TinEye before I post, and those are few.