You guys may need to call foul on this one, and I totally get it. I don’t think this plugin has been updated in about 8 years, but that’s why I’m seeing if you might have some ideas. There doesn’t appear to be a way to include a thumbnail (or any image) in a post excerpt which is why I went hunting for this plugin. It seems to work swimmingly as long as there actually is an image included with the post, although there is a Setting that gets added to specify a default image or tell it not to use a default image. On my home page with my post embed shortcode, however, something seems amiss. You can see it HERE.
It grabs the next image available on the page. If I delete that image, it grabs the next one. Very odd. When I initially said not to use a default image (under Settings) it worked, now suddenly it doesn’t. If you’re interested in taking a peek I’d be happy to provide Admin access.
Thanks for any insight you’re willing and able to provide, and don’t sweat it if this is simply out of your purview.