Getting and Displaying First Attachment

I am currently using the following code in functions.php

<?php
function postimage($size=medium) {
	if ( $images = get_children(array(
		'post_parent' => get_the_ID(),
		'post_type' => 'attachment',
		'numberposts' => 1,
		'post_mime_type' => 'image',)))
	{
		foreach( $images as $image ) {
			$attachmenturl=wp_get_attachment_url($image->ID);
$attachmentimage=wp_get_attachment_image( $image->ID, $size );

echo $attachmentimage;
}
} else {
echo "";
	}
}
?>

and the following code in my themes template files

<?php postimage('thumbnail'); ?>

in order the last attachment image and display it form the current post. Is there any way that this can be altered slightly in order to display the FIRST attachment added.

I have tried reordering them with the gallery feature but this didn’t work.

Thanks