NYPL Digital Collections is a fantastic resource for anyone looking for old photos or vintage drawings to use in their video. It has hundreds of thousands of images scanned by the New York Public Library from posters, leaflets, sketches, playing cards, photos, and countless other sources. I recommend the animal drawings in particular. There are also stereograph images, like this one, which contain two very similar pictures, and when you alternate between them quickly, they give a three dimensional (though somewhat epileptic) effect.
The featured image is something I made when I was using this site to find graphics for a video game I was making with some friends. I found two men with their dogs, and I put the dogs’ heads on the men. There are so many fun little ways of combining these images!
As a final note, it can be difficult to search for things, because it is so massive and doesn’t really sort results by relevance. I’ve found that it helps to skip forward a bit if you run into a book with every page scanned.