Include the first image thumbnail of the "attached images" in the RSS feed

Hi,

I use the following function to get the first image thumbnail of all the “attached images” of a post. I use it in my loop instead of the get_the_post_thumbnail so I don’t have to manually set the thumbnail when creating the post :

<?php
function my_attachment_image_thumb($postid=0, $size='thumbnail', $attributes='') {
	if ($postid<1) $postid = get_the_ID();
	if ($images = get_children(array(
		'post_parent' => $postid,
		'post_type' => 'attachment',
		'numberposts' => 1,
		'orderby' => 'menu_order',
		'post_mime_type' => 'image',)))
		foreach($images as $image) {
			$attachmenthumb=wp_get_attachment_image_src($image->ID, $size);
			?><img src="<?php echo $attachmenthumb[0]; ?>" /><?php
		}
}?>

And I know that the following function puts a thumbnail in the RSS feed, but it uses get_the_post_thumbnail, which I don’t use and returns “empty” :

<?php
function insertThumbnailRSS($content) {
global $post;
if ( has_post_thumbnail( $post->ID ) ){
$content = '' . get_the_post_thumbnail( $post->ID, 'thumbnail' ) . '' . $content;
}
return $content;
}
add_filter('the_excerpt_rss', 'insertThumbnailRSS');
add_filter('the_content_feed', 'insertThumbnailRSS');
?>

So can I somehow merge these two in order to get the first image thumbnail of all the “attached images” of a post inside my RSS feed ?

What would be the code for the function?

Many thanks for your time and help.