As soon as you create a work, it is immediately covered by copyright law, whether you publish it or not. You can publish images with a Creative Commons license, which lets you choose how people are allowed to use it. Can people use it to make money? Can people edit it to create a new image? You decide! The clipart/stock image banks below are full of images that the creators put a Creative Commons Zero license on, meaning anyone can use them for any purpose, without attribution.
If a creator retains copyright of a work, eventually it runs out. If the image was never published, or published in another country then it gets tricky to tell whether it's in the public domain. Most images published in the United States prior to 1923 are in the public domain, because their copyright has expired. Some other more recent images are in the public domain too, but that depends on a long list of variables (see a list here). An easy solution is to search databases of public domain images (bottom of this page), so you already know everything you find is available to republish without permission!
Remember: There are some restrictions when publishing these images, for example when using photos of people clearly featured, or when using an image from a company in a way that implies they endorse something they don't. For more information about restrictions check out this blog post.
All images and videos on Pixabay are released free of copyrights under Creative Commons CC0. You may download, modify, distribute, and use them royalty free for anything you like, even in commercial applications. Attribution is not required.
All photos on Pexels are licensed under the Creative Commons Zero (CC0) license. This means the pictures are completely free to be used for any legal purpose.
All photos published on Unsplash are licensed under Creative Commons Zero which means you can copy, modify, distribute, and use the photos for free, including commercial purposes, without asking permission from or providing attribution to the photographer or unsplash.
Free high-resolution pictures you can use on your personal and commercial projects. Click on an image to download the high-resolution version. New awesomem pictures added weekly! All pictures were photographed by Ryan McGuire and free of copyright restrictions
Public Domain Archive
New 100% free stock photos. Every. Single. Week. Everything you need for your creative projects, all public domain images!
Check out this crowdsourced list of public domain image banks, here are some of our favorites:
British Library Flickr
The British Library has published albums of public domain images on Flickr
New Old Stock
Vintage photos from the public archives. Free of known copyright restrictions. Recapturing history.
Smithsonian's collection of public domain images.
New York Times
A Wikimedia Commons site for public domain images from the New York Times.