My RSS feed thumbnails are not showing

I have a wordpress book blog website, (and I use the extracts feature, featured image and I always have an image placed within my posts at the top of the page).  My RSS feed on Newsblur does not show a thumbnail on my Newsblur screen on my iMac, nor on my iPhone 6 app.  I think it’s a very nice feature and I would love my RSS feed to display a thumbnail.  How do I get the thumbnail to appear?  
Here is a link to one of my pages: and here is my RRS feed link: 

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