To understand the options, you need to have an understanding of copyright and the licensing of photos. Images, just like words, are copyrighted from the moment of their creation. This means that EVERY image on the web has a copyright holder. To be legal, you can't simply Google images, find one you like and use it without permission.
Finding the copyright holder and asking permission can be a full time job. Fortunately, people who care about their pictures being shared tend to fall into two categories: 1) They wish to be paid for the image. Most artists (photographers, painters, print makers, etc.) do that through stock sites. These are sites where you pay by a middleman (iStock, Adobe Stock, Dreamstime, Fotolio, etc.) for a LICENSE to use that image. 2) The copyright holder has decided she will license it under Creative Commons. Creative Commons is an agreed licensing format that allows image owners to retain their copyright while allowing others to copy, distribute, and make some use of their work for free--at least non-commercially.
CCO (sometimes written CC-O). These are the licenses that allow commercial use and, in effect, have marked the images as Public Domain.
Here are some of my favorite searchable image databases that include FREE pictures already vetted for commercial and public domain use.
PIXABY is my favorite. The website offers a large collection of images sorted into categories and tags. It also has an advanced search feature which allows you to narrow your search. You can even search by prominent color in images.
Public Domain Pictures is a large database of royalty free photos that are now part of the public domain. One caveat to this site is that it is a feeder to Shutterstock. That means they display both free and premium images. It is not easy to tell which are which. Even some that are marked Shutterstock will have a free download in a smaller size. However, you can still find some great FREE photos here.
Wikimedia Commons is a public domain and royalty free licensed educational media repository, consisting of both images and videos. It has been around since 2004 and today has more than 36 million files. The searching is a bit archaic, but you can enter terms in the general search window. This is particularly good for historical photos, location photos around the world, and some cultural photos. It is NOT good for general theme searches like romantic couples, scary houses, science fiction or fantasy locations, etc.
Photo Pin was designed for bloggers. What it does is search the Flickr photo database for anything that has a creative commons license. NOTE: The FLIKR terms of service use the creative commons license where attribution is required. So be aware that use of these photos requires you to name the copyright holder. That is easy to do with a tag, a caption, or within the text of a post.
Photographer sites. There are several sites that have imagery by a select group of photographers. These sites are often a way for photographers to highlight their work, while making the image available for use. The images are often excellent. However, the number of images and subject types are usually limited.
Pick Up Image. High quality images from a small group of photographers. Primarily travel/location images and some military images.
Splitshire. Another royalty free database from a smaller group of photographers. Not many thematic shots here. A lot of good general commercial things and some action shots. Better for use as blog posts about specific actions or items. The technology section does have some interesting shots.
If you have other ROYALTY-FREE sites that are your favorites, please share them below.