There are a number of sites such as Pixabay, Pexels, and Unsplash which host CC0 images that are free to use including for commercial purposes, which Steemit is unless you refuse payout for the post. In that case, you may use free for non-commercial use images like I did in this post. As a general rule: When in doubt, assume it's copyrighted. Just about every site has a terms of use or legalese page that specifies their copyright policy. It's still a good policy to source your images, even if they are CC0, so there's no doubt in your reader's mind, and it's nice to credit the content creator even if you're not required to do so. :)