Replace "The Events Calendar"s loading gif

functions.php

if ( ! function_exists('bb_tribe_events_before_html_filter')) {

    /**
     * Replace the events calender loading gif
     * @param $before
     *
     * @return mixed
     * @author Markus Schober
     */
    function bb_tribe_events_before_html_filter($before) {

        $html = $before;

        $doc = new DOMDocument();
        $doc->loadHTML($html);
        $xpath = new DOMXPath($doc);
        $src = $xpath->evaluate("string(//*[@class='tribe-events-ajax-loading']/img/@src)");

        $newSrc = get_stylesheet_directory_uri() . '/assets/images/loading.gif';
        $before = str_replace($src, $newSrc, $before);

        return $before;
    }
    add_filter('tribe_events_before_html', 'bb_tribe_events_before_html_filter');
}