A "reverse image search" is a good way to find out the origins of an image. I think dragging an image into the search bar in Google images usually does the trick.

Since others are suggesting good sources for free-to-use Creative Commons images, I figured I'd hop on that train and post two of my favorite sources.

I use the Prelinger Archives a lot, especially for video work I do. I think they're a great organization, and they have some really fascinating stuff there. I love looking through all the old educational videos and cartoons they have, there's some really useful footage to be found. You can check them out here:

https://archive.org/details/prelinger

The New York Public Library also has lots of great stuff in their Digital Collections. Plenty of it is Creative Commons, and you can specify "Only Public Domain Materials" when making a search. They recently added quite a lot of images, so it's worth browsing through some. I'm currently collecting geological images for a videogame I'm making. You can access their collections here:

https://digitalcollections.nypl.org/

Happy searching!