Wordpress | Show thumbnails in single post pagination

<?php /* Show thumbnails in WordPress Posts Pagination */ ?>
<?php if( $prev_post = get_previous_post() ): ?>
    <div class="nav-box previous">
        <?php $prevthumbnail = get_the_post_thumbnail($prev_post->ID, 'thumbnail' );?>
        <?php previous_post_link('%link',"$prevthumbnail  <p>%title</p>", TRUE); ?>
    </div>
<?php endif; ?>
<?php if( $next_post = get_next_post() ): ?>
    <div class="nav-box next">
        <?php $nextthumbnail = get_the_post_thumbnail($next_post->ID, 'thumbnail' );  ?>
        <?php next_post_link('%link',"$nextthumbnail  <p>%title</p>", TRUE); ?>
    </div>
<?php endif; ?>