<?php $prevPost = get_previous_post(true);
if($prevPost) {?>
<div class="nav-box-previous" style="float:left;">
<?php $prevthumbnail = get_the_post_thumbnail($prevPost->ID, array(100,100) );?>
<?php previous_post_link('%link',"$prevthumbnail %title", TRUE); ?>
</div>
<?php }?>
<?php $nextPost = get_next_post(true);
if($nextPost) { ?>
<div class="nav-box-next" style="float:right;">
<?php $nextthumbnail = get_the_post_thumbnail($nextPost->ID, array(100,100) ); ?>
<?php next_post_link('%link',"$nextthumbnail %title", TRUE); ?>
</div>
<?php }?>