Just because they have posted it does not mean they are copyright free. The people who posted it don’t necessarily own the usage rights, so they may not have made them.
I recommed reverse searching these on google and see where the orginal image is from. If it doesn’t say “free for commercial use” you cannot use it in your story.
You can search copyright-free images for backgrounds and overlays on pexels, pixabay and unsplash. There are high resolution images there. Be careful about what you use in your stories.