Image galleries are a great way to share groups of pictures on your WordPress site. The Create Gallery feature of the WordPress media uploader allows you to add a simple image gallery to pages or posts on your site.  This post will explain you how to change wordPress default gallery output

There’s a filter to post_gallery which you can use to customize all default WP galleries.

Following is the code I found last day and it help us to change wordPress default gallery output according to our wish.

Just paste below code to your functions.php file and modify to adapt it to your need.

 * Change WordPress default gallery output
add_filter('post_gallery', 'ct_post_gallery', 10, 2);
function ct_post_gallery($output, $attr) {
    global $post;
    if (isset($attr['orderby'])) {
        $attr['orderby'] = sanitize_sql_orderby($attr['orderby']);
        if (!$attr['orderby'])
        'order' => 'ASC',
        'orderby' => 'menu_order ID',
        'id' => $post->ID,
        'itemtag' => 'dl',
        'icontag' => 'dt',
        'captiontag' => 'dd',
        'columns' => 3,
        'size' => 'thumbnail',
        'include' => '',
        'exclude' => ''
    ), $attr));
    $id = intval($id);
    if ('RAND' == $order) $orderby = 'none';
    if (!empty($include)) {
        $include = preg_replace('/[^0-9,]+/', '', $include);
        $_attachments = get_posts(array('include' => $include, 'post_status' => 'inherit', 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => $order, 'orderby' => $orderby));
        $attachments = array();
        foreach ($_attachments as $key => $val) {
            $attachments[$val->ID] = $_attachments[$key];
    if (empty($attachments)) return '';
    // Here's your actual output, you may customize it to your need
    $output = "<div class=\"slideshow-wrapper\">\n";
    $output .= "<div class=\"preloader\"></div>\n";
    $output .= "<ul data-orbit>\n";
    // Now you loop through each attachment
    foreach ($attachments as $id => $attachment) {
        // Fetch the thumbnail (or full image, it's up to you)
//      $img = wp_get_attachment_image_src($id, 'medium');
//      $img = wp_get_attachment_image_src($id, 'my-custom-image-size');
        $img = wp_get_attachment_image_src($id, 'full');
        $output .= "<li>\n";
        $output .= "<img src=\"{$img[0]}\" width=\"{$img[1]}\" height=\"{$img[2]}\" alt=\"\" />\n";
        $output .= "</li>\n";
    $output .= "</ul>\n";
    $output .= "</div>\n";
    return $output;


So you can change this code to change the default gallery output.