Link featured image to lightbox gallery of post images

What I want to do: when you click the featured image on a post a lightbox opens and shows all the images added as attachment to the post (the images when you check under the ‘gallery’ section of the upload module)

So far I got the featured image working and it opens in a lightbox (I’m working with prettyPhoto). I also was able to query for the attached images and got them to display on the page.

<?php  $args = array(
   'post_type' => 'attachment',
   'numberposts' => -1,
   'post_status' => null,
   'post_parent' => $post->ID
  );
  ?>
<?php
	$attachments = get_posts( $args );
	if ( $attachments ) {
	foreach ( $attachments as $attachment ) {
	echo '<p>';
	echo wp_get_attachment_image( $attachment->ID, 'thumb' );
	echo '</p>';

		}
	}?>

What I can’t figure out is:
a) how to link those images to their url so they can be shown in a lightbox
b) how to make sure only the ‘featured image’ (or the first image) is displayed but links to the other images in the lightbox.

I’m having a hunch that I’m going about this the wrong way. Anyone have suggestions or pointers? Thanks!