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add_filter('post_gallery', 'my_post_gallery', 10, 2);
function my_post_gallery($output, $attr) {
    global $post;

    if (isset($attr['orderby'])) {
        $attr['orderby'] = sanitize_sql_orderby($attr['orderby']);
        if (!$attr['orderby'])
            unset($attr['orderby']);
    }

    extract(shortcode_atts(array(
        '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 '';
$lines_count = count($attachments);
if($lines_count%3 == 0){
	$lines_count = 3;
}
if($lines_count%4 == 0){
	$lines_count = 4;
}
if($lines_count%5 == 0){
	$lines_count = 5;
}
    // Here's your actual output, you may customize it to your need
    $output = "<div class=\"gallery-wrapper\">\n";
    $output .= "<ul class=\"small-block-grid-1 medium-block-grid-".$lines_count." large-block-grid-".$lines_count."\">\n";
    // Now you loop through each attachment
    foreach ($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, 'thumbnail-gallery');
        $img_big = wp_get_attachment_image_src($id, 'large');

        $output .= "<li>\n";
        $output .= "<a href=\"{$img_big[0]}\" class=\"fancybox\"  rel=\"single_gallery\">\n";
        $output .= "<img src=\"{$img[0]}\" width=\"{$img[1]}\" height=\"{$img[2]}\" alt=\"\" />\n";
        $output .= "</a>\n";
        $output .= "</li>\n";
    }

    $output .= "</ul>\n";
    $output .= "</div>\n";

    return $output;
}