Wrong Thumbnails when Sharing Posts

In the header of my index page, I added this line:

<link rel="image_src" href="https://www.cyberquill.com/fbthumb.png" />

Depending on the site where I want to share my index page, this picture either doesn’t show up at all (and a random picture gets selected instead), or it appears somewhere under “Choose a Thumbnail.”

The header of my single post page contains no ‘link rel=”image_src”‘ instruction in hopes that the post’s “Featured Image” will be selected as this post’s default thumbnail, which generally works EXCEPT when sharing on Facebook, where it works sometimes but not other times.

When sharing on Facebook, my most recent post, for instance, stubbornly refuses to display the Featured Image I selected, instead only allowing one of three post-unrelated images to be chosen as thumbnail.

What am I doing wrong?