I write blog posts and create an ever-expanding amount of marketing content for our company. I often have to find free stock images, and I am impressed with how good the public domain offerings are getting. Here are the sources that seem to rise to the top.
Things to watch out for
Before getting to the sites, I should note that copyright law is important to follow and there are certainly different forms of "free" license. Public Domain Sherpa and Creative Commons provide great information about license types. Generally speaking, I prefer "free to use, including modification, for commercial use, without attribution" but I consider attribution if that's all that's available. In any case, it’s important to know which license you are using. "CC0" is nice as it doesn't require attribution.
Enough watching out, here are the sites...