function bm_my_post_thumbnail_html( $html, $post_id, $thumbnail_id, $size = '' ) {

    if ( empty( $html ) ) {

        $values = get_children(
            array(
                'post_parent' => $post_id,
                'post_type' => 'attachment',
                'post_mime_type' => 'image',
                'order' => 'ASC',
                'orderby' => 'menu_order',
                'numberposts' => 1,
            )
        );

        if ( $values ) {
            foreach ( $values as $child_id => $attachment ) {
                $html = wp_get_attachment_image( $child_id, $size );
                break;
            }
        }

    }

    return $html;

}
add_filter( 'post_thumbnail_html', 'bm_my_post_thumbnail_html', 10, 4 );