Unable to get thumbnails to appear in Readers

I’m not able to get any pics (thumbnails) from my posts to show up in feed readers.

This is an example of what I’m trying to accomplish. See how the thumbnail appears on the left of each listing.
https://www.zspotlight.com/blog/test_reader.html

I’m able to accomplish everything else but can’t get the pic to appear.

Thanks for any help.