So ...

Method #2: Explicitly tell Facebook to use a thumbnail of your choice

Again, there are many different ways to do this ... SEO plugins, social media plugins, etc.

I'm using YOAST SEO plugin which provides a clear and simple solution.

Open your post in the post editor, scroll down to the YOAST SEO meta box below the post editor and select the social tab. Simply add your Facebook thumbnail image there, and this will fix the issue in most cases:

And if it continues to show the incorrect thumbnail, you may have to reset the Facebook cache.

OK. Let's move forward to see the last method ...