WP Gallery: link image to parent page

I’m using the wordpress gallery in my page (no plugin). The images in gallery link to a page where the image is displayed. If you click the image it will link to the next image in the gallery. I don’t want it to link to the next image, I want it to link back to the parent page (the gallery). I know it must be changing some code in the attachment.php file, but how to do this?

<?php if ( wp_attachment_is_image() ) :

	$attachments = array_values( get_children( array( 'post_parent' => $post->post_parent, 'post_status' => 'inherit', 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => 'ASC', 'orderby' => 'menu_order ID' ) ) );

	foreach ( $attachments as $k => $attachment ) {

		if ( $attachment->ID == $post->ID )

			break;
	}

	$k++;

	// If there is more than 1 image attachment in a gallery

	if ( count( $attachments ) > 1 ) {

		if ( isset( $attachments[ $k ] ) )

			// get the URL of the next image attachment

			$next_attachment_url = get_attachment_link( $attachments[ $k ]->ID );

		else

			// or get the URL of the first image attachment

			$next_attachment_url = get_attachment_link( $attachments[ 0 ]->ID );

	} else {

		// or, if there's only 1 image attachment, get the URL of the image

		$next_attachment_url = wp_get_attachment_url();

	}

?>

						<p class="attachment"><a href="<?php echo $next_attachment_url; ?>" title="<?php echo esc_attr( get_the_title() ); ?>" rel="attachment"><?php

							$attachment_size = apply_filters( 'slidingdoor_attachment_size', 900 );

							echo wp_get_attachment_image( $post->ID, array( $attachment_size, 9999 ) ); // filterable image width with, essentially, no limit for image height.

						?></a></p>