Know, that as an American citizen you are protected by the laws of the land. One of the most comprehensive laws is that governing copyright, trademark, and service mark.
Familiarize yourself with The Lanham Act that allows others fair use of your work. Know that copyright also has to be defended in court if challenged - and that can be an expensive and lengthy battle.
There are two low cost ways to copyright an image or written work:
Print it, or more preferably hand write it/draw it.
Put it in a manilla legal envelope, seal, and mail to yourself from a post office with a certified return receipt. On the receipt put copyrighted © poetry by [your name]
Publish it, anywhere, especially online because it will have a timestamp.
Include your name and the © symbol and the current year.
If you regularly publish online or have your own blog, your site is protected by the nature of publishing. You do not have to include a copyright notice but it doesn't hurt if you wish to not be a victim of plagiarism or piracy.
Social media and search engines are actually very responsive to takedown notices if you can prove ownership.