CC0 images are Creative Commons 0 (zero). Defined by wiki as, ” Once the creator or a subsequent owner of a work applies CC0 to a work, the work is no longer his or hers in any meaningful sense under copyright law. Anyone can then use the work in any way and for any purpose, including commercial purposes, subject to other laws and the rights others may have in the work or how the work is used. Think of CC0 as the “no rights reserved” option.”
some CC0 image websites are listed below. Be aware if they request that you attribute the website or photographer. Some images do, others don’t
pexels.com
unsplash.com
burst
pikwizard
pixabay
pixnio.com
dreamstime (search for attribution if you want free.)