Getting attachment images in the sidebar

I am trying to get the thumbnails from a categories posts into the sidebar (outside the loop) but can’t seem to work it. I am trying to use the following function:

<?php
# Displays post image attachment (sizes: thumbnail, medium, full)
function side_th_image($postid=0, $size='thumbnail', $attributes='') {
if ($postid<1) $postid = get_the_ID();
if ($images = get_children(array(
'post_parent' => $postid,
'post_type' => 'attachment',
'numberposts' => 1,
'post_mime_type' => 'image',)))
foreach($images as $image) {
$attachment=wp_get_attachment_image_src($image->ID, $size);
?><?php echo $attachment[0]; ?><?php
}
}
?>

In the following sidebar code:

<h2>From the collection</h2>

<ul>
  <?php  global $post;
         $myposts = get_posts('category=1&numberposts=5&orderby=rand');
         foreach($myposts as $post) : ?>
            <li><img src="<?php side_th_image(); ?>"></li>
         <?php endforeach; ?>
</ul>

Any ideas where I’m going wrong? This works fine when I call the code into the loop.