Function to check if a event have a featured image.

function check_featured_image(){
         global $post;
         $featured_image = tribe_event_featured_image($post->ID);
         if($featured_image){
             return $featured_image;
         }else{
            set_post_thumbnail( $post->ID, '0000' ); // here the ID of the image that you want to use as default.
            $featured_image = get_the_post_thumbnail($post->ID);
            return $featured_image;
         }

}