That's the default behaviour for facebook share button.

For twitter add this function in your theme or child-theme functions.php file:

add_filter('stec_get_event_og', function($og, $event, $offset) {
    $image = '';
    if ($event->get_images()) {
        $images = $event->get_parsed_images();
        $image  = $images[0]->src;
    }
    return array_merge($og, array(
        'twitter:card'        => 'summary_large_image',
        'twitter:title'       => $event->get_title(),
        'twitter:description' => $event->get_description_short(),
        'twitter:image'       => $image,
    ));
}, 10, 3);

I'll include the twitter og data in the next update.