Use the jQuery thickbox plugin for your WordPress galleries (add code to your theme's function.php file)

add_thickbox_attr.php

function add_thickbox_attributes_gallery_link( $anchor_tag, $image_id ) {
    $image = get_post( $image_id );
    if( isset($image->post_parent)) {
        $rel = '<a class="thickbox" rel="attached-to-'.intval($image->post_parent).'"';
        $anchor_tag = str_replace('<a', $rel, $anchor_tag);
    }
    return $anchor_tag;
}
add_filter('wp_get_attachment_link', 'add_thickbox_attributes_gallery_link', 1, 2);

load_thickbox.php

function load_thickbox() {
	if (is_single() || is_page()) {
		wp_enqueue_script('thickbox');
		wp_enqueue_style('thickbox');
	}
}
add_action('wp_enqueue_scripts', 'load_thickbox');