Please note that image is fetched only if "the rss feed has the image attached to it" For example blogspot RSS feed URLs have images embedded in the post.
Wordpress feeds dont have post images embedded in it. So the plugin wont show any thumbnail. Inorder to add post thumbnail images to the feeds, you hsould use this plugin : http://wordpress.org/extend/plugins/wp-rss-images
After that Super RSS Reader will display the thumbnails of your wordpress site.