New issue with images on posts

I’ve started having an odd issue. When I post a link to my blog on Facebook or other places certain posts stopped pulling any thumbnails or pictures from the post itself. Could someone tell me what might cause this? My images have all worked fine up until a week or so ago. There are plenty of images to choose from on each post. This is an example of a post that will not show pull an image on social networking spots:
https://ourpeacefulplanet.com/2014/06/30/bell-jar-display-tutorial/

Thank you!
Nancy