add_filter('tribe_events_event_thumbnail', 'custom_default_event_image', 10, 2);
function custom_default_event_image($image, $event) {
    if (empty($image)) {
        return '<img src="' . esc_url(get_stylesheet_directory_uri() . '/images/default-event.jpg') . '" alt="Default Event Image" />';
    }
    return $image;
}