The US Copyright office does not maintain a list of media, text, music, or images in the public domain. While you can register your copyrighted work, there is no office that allows you to share your work freely. Enter Creative Commons, an organization dedicated to protecting copyright by making it easier for creators to share their work while restricting what can be done to it.
Here is the Creative Commons video to explain:
You’ll notice, this website is licensed with Creative Commons under an “Creative Commons Attribution-ShareAlike 4.0 International license”. Basically, if you find something useful on this site I want you to use it! But if it’s something I wrote or created (like this paragraph) I want you to give credit where it’s due.
This is really a good rule anywhere on the web- don’t be a jerk, give credit to the author/creator. Especially with image sharing sites, this can get tricky. In presentations and publications always cite your sources. On the interwebs play nice, and link back to the source when posting on Pinterest/Twitter/Facebook/whichever social media you choose.
This work by https://u.osu.edu/rusnak.38 is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.