[Image Gallery] How to link the main image to next image

Hi all,
take a look at this Matt’s page: http://ma.tt/2008/12/sydney-zoo-opera-house/mcm_5817/

The big image on the top is linking to the next image in the gallery, not to the full-resolution file version.

My image.php has this code to link to the image itself:

<div class="attachment-image">
<a href="<?php echo wp_get_attachment_url($post->ID); ?>">
<?php echo wp_get_attachment_image($post->ID, 'full'); ?>
</a>
</div>

How can I modify this to link the main image to the next image in gallery?