Okay, I don't understand what is happening here.
Initially, I started this exercise by copying code into functions.php. This worked but I was unhappy using unattributed code I didn't understand. So, I downloaded this plugin and added a new post. When the post eventually appeared in Feedly I saw the post but no image and I wrote the first feedback.
I then deactivated this plugin and installed another and added a new post. Now, whilst I'm waiting for the latest post to arrive I see the "missing" image appear. I do a refresh and it appears again but only after quite a delay. The image full size is only 168x112 pixels.
Now, I do not know if the "missing" image was there all along and I was too impatient or, when I click "refresh", the posts are downloaded again and the image is coming from the later plugin.
What do you think?