Images shown when sharing post on Facebook inconsistent

I use the Hupso sharing plugin, and when I go to share a post on Facebook, sometimes there will be several images to choose from, sometimes only one. When it’s only one, it’s the image I have chosen in the “Social” tab of Yoast WordPress SEO, my SEO plugin, for the image to be used “if the post/page being shared does not contain any images.” This is the image that gets listed in the html for meta property=’og:image’.

I also use the Post Avatar plugin to display the author’s image in the post. This image will sometimes be found when sharing the post on FB, but not always. And when there is more than one image available, there will often be up to 5 or 7 of them, including the website logo, Post Avatar image, and images in the sidebars.

So, my question is, how can I force the Post Avatar image to be available when sharing a post?

I’ve compared the Source html from two posts, both with only the Post Avatar image in the post, one that Facebook found several images for, and the other that it only found the one image. I can find no difference in the html.

An example page that has this problem is https://www.humortimes.com/20172/florida-7-rows-of-7-stars/

(Note: When I insert another image in the post, that image always available when sharing.)