Hi Joseph, all you need to do is add a featured image for your event(s) and the thumbnail should automatically appear in the tooltip. Have you tried that?
It looks like you also inserted that thumbnail into the event content. Have you tried removing it? The plugin automatically insert the event featured image into the tooltip and top of the single event view. If you want to modify what shows up in the single event view, just make a copy of /wp-content/plugins/the-events-calendar/views/single.php, place in an ‘events’ folder in your theme and edit away!