Related posts below posts are shown to reduce the bounce rate and increase the internal linking . There are many wordpress plugins already available in the market but many of them lacks use the common thing (Thumbnail). Showing thumbnail in your related posts make them more appealing and increase the chances of click.
Wp-thumbie is one such Wordpress plugin , which shows related posts with thumbnail below post. This plugin comes with a plugin option page to make many changes.
This plugin is easy to use and configure. Infect simply installing and activating the plugin will start showing related posts with thumbnail.
Check the screenshot of plugin in action :
