Debugging in Facebook
In some cases, when you post the URL on FB, the image doesn’t automatically show up.
It’s a known bug being repaired. In the meantime, you can use the FB debugger to get the image showing.
Before doing your post, paste the article URL here: https://developers.facebook.com/tools/debug/
Click debug and scrape info, and then go back to Facebook and do your post.
The image should show automatically.