Function not working on archive pages

Hi,
I have a problem – I have a function that takes the first attached image of a custom post type and renders it. It works great on all pages except one type – the category archive pages. There it doesn’t work at all. Any ideas?
It is exactly the same code before/after the function on the category pages as for example the home page.

Here is my function:

//Thumb img
function get_first_image($id) {
	$PostID = $id;
	$all_images =& get_children('post_type=attachment&post_mime_type=image&post_parent=' . $PostID );
	if($all_images) {
	$arr_of_all_images = array_keys($all_images);
	$firstImage = $arr_of_all_images[0];
	$thumb_url = wp_get_attachment_thumb_url($firstImage);
	$First_thumb_image = '<a href="' . get_permalink() . '">' .
	'<img src="' . $thumb_url . '" width="150" height="150" />' .
	'</a>';
	echo $First_thumb_image;
	}
}