Previous and Next posts

Try this (untested):

<?php 
    $prevPost = get_previous_post();

    foreach ($prevPost as $postPrev) {
        if ( has_post_thumbnail($postPrev->ID)) {
            $prevThumbnail = get_the_post_thumbnail($postPrev->ID, array(150,150) );
            previous_post_link( '%link', $prevThumbnail );
        }
    }
?>