A public domain work is a creative work that is not protected by copyright and which may be freely used and adapted by anyone. The reasons a work is in the public domain may include:
This image of Carina Nebula was taken by NASA's James Webb Telescope. The photo is owned by NASA, a government entity, and is therefore in the public domain.
This image from 1900, taken by Theresa Babb of a group of friends picnicking in Camden, ME has surpassed its copyright limit (95 years) and is no longer protected by copyright.
The person who took this photo of a busy New York City scene waived their copyright and put it in the public domain for anyone to use. Image by Pexels from Pixabay
Creative Commons is a nonprofit organization that enables the sharing and use of creativity and knowledge through creative commons licenses. These licenses allow creators greater control over their ability to share their work and allow others to use it. Creative Commons is a great resource for finding materials to utilize as well as sharing your work with the world. The website contains a section dedicated to Public Domain Tools.