<?php

add_filter('tribe_events_before_html', 'my_tribe_events_before_html', 100);
function my_tribe_events_before_html($html) {
    $events_label_plural = tribe_get_event_label_plural();
    $before = stripslashes( tribe_get_option( 'tribeEventsBeforeHTML', '' ) );
    $before = wptexturize( $before );
    $before = convert_chars( $before );
    $before = wpautop( $before );
    $before = do_shortcode( stripslashes( shortcode_unautop( $before ) ) );
    $before = '<div class="tribe-events-before-html">' . $before . '</div>';
    $before = $before . '<span class="tribe-events-ajax-loading"><img class="tribe-events-spinner-medium" src="' . 'path/to/your-custom.gif' . '" alt="' . sprintf( esc_html__( 'Loading %s', 'the-events-calendar' ), $events_label_plural ) . '" /></span>';

    return $before;
}