dynamically linking post images to full size

Several contributors have added image(s) to hundreds of posts on our blog. It’s been requested that when a post image is clicked, it pops in a lightbox with the full-size image. I’ve piece-mailed together some code to find the image-link in each post, add rel=”lighbox[post-slug]” to hook in a lightbox, and the only challenge yet is to change the attachment image to point to the full-size image so the lightbox knows what to do.
Basically, I am unable to get the attachment-image link/id dynamically.
here is the code I have so far:

<?php
/*
change all post image links to link to larger image
*/
add_filter( 'the_content', 'attachment_image_link_change_filter' );
function attachment_image_link_change_filter( $content ) {

$content = preg_replace(
 	array('{<a(.*?)[^>]*><img}','{wp-image-[0-9]*" /></a>}'),
  	array('<a href="%IMGLINKLG%"><img','" /></a>}'),
	$content);
 $content = str_replace("%IMGLINKLG%", wp_get_attachment_url($id), $content);
    return $content;
}

/*
Add lightbox
*/
add_filter('the_content', 'addlightboxrel_replace');
function addlightboxrel_replace ($content)
{	global $post;
	$pattern = "/<a(.*?)href=('|\")(.*?).(bmp|gif|jpeg|jpg|png)('|\")(.*?)>/i";
  	$replacement = '<a$1  rel="lightbox[%LIGHTID%]" title="%TITLE%" href=$2$3.$4$5$6>';
    $content = preg_replace($pattern, $replacement, $content);
	$content = str_replace("%LIGHTID%", $post->post_name, $content);
	$content = str_replace("%TITLE%", $post->post_title, $content);
    return $content;
}
?>

wp_get_attachment_url($id) is the piece that needs to change but not sure what else to do….