Every post that I publish here on my site has a featured image associated with it. I try to use images that reflect a little about what I am writing and most of the time I think I am successful.
These images are actually very easy to come by and are free to download and use. Two sites that use almost exclusively are Unsplash and Pexels. The licensing for both is so simple and can be found here and here.
Both Unsplash and Pexels do not require you to give credit/attribution to the photographer or the site. However it is greatly appreciated and encourages the photographers to provide more of their work.
So if you are looking for great free stock images, then look no further than Unsplash and Pexels.