How to use Public Domain images and three sites where you can find them.
If your looking for a historical edge to your marketing pieces, go on a hunt for images within the Public Domain. These photos and illustrations fall under the CC0 License. CC0 stands for "Creative Commons" and it means that an image under this license is free for personal and commercial use with no attribution required.
There are hundreds of thousands of free use graphics out in the world and I recently stumbled across Raw Pixel. This site has wonderful historical photographs and illustrations that can be used with a CC0 License.
Another great source for these types of images is the New York Public Library. They have over 180,000 images in their public domain database. That's right, one hundred eighty THOUSAND! A great feature they've added to their collection is the ability to search by color. So you can find a slough of images that match your brand!
The third site I'd like to direct your attention to is publicdomainpictures.net Granted, the user experience on this site isn't the best and you have to be careful about hidden ads. You can distinguish these by the word "AD" in the top right hand corner of the thumbnail. But if you can wade through the issues, they have some pretty great, old travel posters.
So if you're looking for some historical and old-world graphics to enhance your social posts, check out the Public Domain sections of these site.
How should you use public domain images?
Well, pretty much any way you want. According to the official creative commons license:
The person who associated a work with this deed has dedicated the work to the public domain by waiving all of his or her rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
You can copy, modify, distribute and perform the work, even for commercial purposes, all without asking permission.
CC0 tends to apply to work that has been created in the past; usually 100+ years have elapsed and the original copyright has expired. But also take into account that you can in no way make it seem like you have express permission to use the artwork or claim that originator has "endorsed" you. You can view the actual CC0 license verbiage here.