I use the following code for displaying the previous post thumbnail with links:
<?php
$prevPost = get_previous_post(true);
$prevThumbnail = get_the_post_thumbnail($prevPost->ID, array(150,150) );
previous_post_link( '%link', $prevThumbnail );
?>
The problem is that I get the correct link for the previous post but the thumbnail that is displayed is of the current post and not of the previous post. I am using this for a single template of a custom post type.