function add_lightbox_script() {
    wp_enqueue_script( 'fancybox-js', 'https://cdn.jsdelivr.net/npm/@fancyapps/fancybox@3.5.7/dist/jquery.fancybox.min.js', array('jquery'), '3.5.7', true );
    wp_enqueue_style( 'fancybox-css', 'https://cdn.jsdelivr.net/npm/@fancyapps/fancybox@3.5.7/dist/jquery.fancybox.min.css', array(), '3.5.7' );
}
add_action( 'wp_enqueue_scripts', 'add_lightbox_script' );

function add_lightbox_script_inline() {
    ?>
    <script>
        jQuery(document).ready(function($) {
            $('a[href*=".jpg"], a[href*=".jpeg"], a[href*=".png"], a[href*=".gif"]').attr('data-fancybox', 'gallery');
        });
    </script>
    <?php
}
add_action( 'wp_footer', 'add_lightbox_script_inline' );