I have had a blog for over a year and have had no image difficulties until recently - past month.
I add images from a number of sources, mostly Flickr, or my media library. Most of the time now, but not always, when I try to post my article in social media the thumbnail is not being pulled.
I use Google Chrome and tried other browsers. That did not seem to make a difference.
Sometimes I pull the HTML code from the image's Flickr page and insert it into the article always at the top. Otherwise I ma download it, upload it to my media library and add from there. I also have a Flickr plugin I sometime use.
I have no idea where to look for the problem or a solution.