Public domain images are images that are free to use or have their own free to use license.
Images are public domain because:
Public domain images are free to use in any context but it is still good practice to attribute the creators where you can.
FOR MORE INFORMATION ON ATTRIBUTING IMAGES SEE THE GIVING CREDIT FOR IMAGES PAGE
There are lots of places to find images that you can use for your work. These two are a great place to start searching:
Pixabay is great, especially for photos but you can also search by vector graphics and illustrations. No attribution is required but it's good practice to at least attribute the source.
Pexels is another great place to search for free images. No attribution is required but it's good practice to at least attribute the source.