Post thumbnails outside the loop

My sidebar is as follows:

<div id="sidebar">

	<h2>Beste Aminos</h2>

	<ul id="top-producten">
		<?php $rand_posts = get_posts('numberposts=5&orderby=rand'); foreach ($rand_posts as $post) :?>

		<li>
			<h2><a href="<?php the_permalink(); ?>"><?php the_title();?></a></h2>
			<p><a href="<?php the_permalink(); ?>"><?php $key="subtitle"; echo get_post_meta($post->ID, $key, true); ?></a></p>
			<?php get_the_post_thumbnail() ?>
			<?php wp_gdsr_render_article();?>
			<p class="meer"><a href="<?php the_permalink(); ?>">Meer informatie & bestellen</a></p>
		</li>

	<?php endforeach; ?>

	</ul>

</div><!--/#sidebar-->

This is probably the most i can make from my php skills.

The problem occurs at the post_thumbnail: The sidebar repeatedly displays the thumbnail of the active page instead of the thumbnails of from the linked posts.

I tried some solutions I found on the net, but none of them are working.

Who could help me out, please?