All images on the web are protected by copyright. Even if the copyright is not mentioned, you should assume they are. And using these images without permission is illegal.
While you can buy high-quality and royalty-free images from stock photo websites like Adobe Stock or Shutterstock, not everyone has the budget to purchase licensed photographs and images.
That’s when images with public domain and the CC0 license come in handy.
Public domain is the term used to describe works whose licenses have expired or are explicitly released with no restriction on their usage.
CC0 is a license that allows copyright owners to release their works with no restrictions at all.
Anyone can use images under the public domain or CC0-License for any purpose. As a website owner, you can use these images in your blog posts, featured images, sliders, image galleries, backgrounds, and basically anywhere else you want.