Lead Developer
Hi Shirley,
I think for an image to automatically display, there has to be an image that is at least 200x200px on the page.
Otherwise, you can use a plugin like this: https://en-ca.wordpress.org/plugins/wonderm00ns-simple-facebook-open-graph-tags/
Or you can choose your own photo to upload when adding the post (this is what I do on our FB page).