Force thumbnail of first image attachment into excerpt

I’m looking for a way for WP to save the image code of the first image attachment into the excerpt field when a post is published. Any idea how I could accomplish this?

So basically the excerpt field would contain something like:

<img src="http://testing.com/wp-content/uploads/2009/09/EastBeach21-50x50.jpg" width="50" height="50" alt="Test" title="Test" />

upon publishing the post.

I have this code that does it when you hit save draft, but I’m using a combination of Postie and DagonDesigns draft schedule plugin, and need it to add it to the excerpt without needing to hit save draft, since the scheduler will publish automatically. This code is a little buggy too btw

function pb_insert_excerpt(){
	$post_data = &$_POST;
	$post_id = $post_data['ID'] ;
	$post_title = $post_data['post_title'];
	$post_excerpt = $post_data['post_excerpt'];
	$arrImages =& get_children('post_type=attachment&post_mime_type=image&post_parent=' . $post_id);
	if($arrImages) {
		$arrKeys = array_keys($arrImages);
		$iNum = $arrKeys[0];
		$sThumbUrl = wp_get_attachment_thumb_url($iNum);
		$thumbWidth = get_option("thumbnail_size_w");
		$thumbHeight = get_option("thumbnail_size_h");
		$sImgString = '<img src="' . $sThumbUrl . '" width="'.$thumbWidth.'" height="'.$thumbHeight.'" alt="'.$post_title.'" title="'.$post_title.'" />' ;
		$existing_img = strstr($post_excerpt, 'jpg');
		if($post_data['post_excerpt'] = isset($post_data['excerpt'])) {
			if ($existing_img) {
				return $post_excerpt;
			} else {
				return $sImgString;
			}
		}
	}
}

Thank you