Featured Image Link to next post?

Question

Something like this should work for your single.php template:

<?php
$next_post = get_adjacent_post( false,'',false );
if( isset($next_post->ID) ):
    $next_id = $next_post->ID;
else:
    $next_post = new WP_Query( 'posts_per_page=1&post_type=photo&order=ASC' );
    $next_id = $next_post->post->ID;
endif;
?>
    <a href="https://wordpress.stackexchange.com/questions/27977/<?php echo get_permalink( $next_id ); ?>">
        <?php echo get_the_post_thumbnail( $next_id, 'thumbnail' ); ?>
    </a>