function pippin_gallery_shortcode( $atts, $content = null ) {
 
	extract( shortcode_atts( array(
      'size' => ''
      ), $atts ) );
 
	$image_size = 'medium';
	if($size =! '') { $image_size = $size; }
 
	$images = get_children(array(
		'post_parent' => get_the_ID(),
		'post_type' => 'attachment',
		'numberposts' => -1,
		'orderly' => 'menu_order',
		//'exclude' => get_post_thumbnail_id(), -- uncomment to exclude post thumbnail
		'post_mime_type' => 'image',
		)
	);
 
	if($images) {
		$gallery = '<ul class="gallery clearfix">';
		foreach( $images as $image ) {
			$gallery .= '<li>';
				$gallery .= '<a href="' . wp_get_attachment_url($image->ID) . '" rel="shadowbox">';
					$gallery .= wp_get_attachment_image($image->ID, $image_size);
				$gallery .= '</a>';
			$gallery .= '</li>';
		}
		$gallery .= '</ul>';
 
		return $gallery;
	}
 
}
add_shortcode('photo_gallery', 'pippin_gallery_shortcode');

UPDATE: might be easier or faster to use this now (also courtesy Pippin): http://pippinsplugins.com/get-all-attached-media/

**courtesy Pippin Williamson (http://pippinsplugins.com/image-gallery-short-code-using-post-attachments/)

use shortcode [photo_gallery] to display the gallery

Wordpress Image Gallery Short Code Using Post Attachments

php, image, wordpress

PHP