thumbnails only showing generic images

We have installed this plugin on our site, but the related posts thumbnails are the generic ones and NOT the featured posts thumbnails that we have set. We can’t go through and set the custom field for all of our posts and need a way for the image to show up. An example of what we see is here:

http://animalfair.com/big-dog-little-dog-5-travel-tips/

https://wordpress.org/plugins/related-posts-by-zemanta/