Simple WordPress image gallery shortcode. Add to functions.php. Images categorised as 'Gallery' will be fetched. Data attribute is for Featherlight lightbox.

Simple WordPress image gallery shortcode.php

<?php

//Uncomment to enable shortcodes in widgets
/* add_filter('widget_text', 'do_shortcode'); */

// Add Shortcode for Image Gallery Widget
function image_gallery_shortcode() {
    ob_start();
        $args = array(
            'post_type' => 'attachment',
            'post_mime_type' => 'image',
            'orderby' => 'rand',
            'order' => 'desc',
            'posts_per_page' => '9',
            'post_status'    => 'inherit',
            'category_name' => 'Gallery'
             );
         $loop = new WP_Query( $args );

        while ( $loop->have_posts() ) : $loop->the_post();
            $thumb = wp_get_attachment_image_src( get_the_ID() );
            $large = wp_get_attachment_image_src( get_the_ID(),'large' );
            echo "<a href='' data-featherlight='" . $large[0] . "'><img src='" . $thumb[0] . "'></a>";
        endwhile;

    return ob_get_clean();
}
add_shortcode( 'image-gallery', 'image_gallery_shortcode' );