<div id="next-prev-links">
<?php
$previous_post = previous_post_link('%link', '', TRUE);
$next_post = next_post_link('%link','', TRUE);
$image_prev = get_post_meta( $previous_post->ID, 'image', true);
$image_next = get_post_meta( $next_post->ID, 'image', true);
?>

<?php if ( $image_prev != '' ) : ?>
  <img src="<?php echo $image_prev; ?>" alt="" />
<?php endif; ?>

<?php if ( $image_next != '' ) : ?>
  <img src="<?php echo $image_next; ?>" alt="" />
<?php endif; ?>

<?php previous_post_link('%link', 'Previous post', TRUE); ?>
<?php next_post_link('%link', 'Next post', TRUE); ?>
</div>