Thumbnails won’t display

I’m using Version 4.0.13 of the plugin. I have a widget configured on the bottom of the front page to show 4 popular posts. I am using custom HTML with this markup:

<li class="one-fourth">{thumb} {text_title}<br><a href="{url}">Read more ></a></li>

When I use the inspector on the front page, there is no image being generated at all.

I tried turning off custom HTML, and still no thumbnail is displayed. If I try to check the box to display a thumbnail, the box unchecks itself when I click the save button on the widget

The page I need help with: [log in to see the link]