Custom field thumbnail in link for Previous Next Posts

In a bit of a follow-up to a previous question, I am currently using this bit of code on my main page to output thumbnails above each post title from a custom field, unless there is none, in which case I use a default:

<a href="<?php the_permalink(); ?>" title="" rel="bookmark">
<?php $thumbnail = get_post_meta($post->ID, 'Thumbnail', true); if ($thumbnail) { ?>
<img src="<?php echo get_post_meta($post->ID, "Thumbnail", true); ?>" alt="<?php the_title(); ?>" title="<?php the_title(); ?>" />
</a>

<?php } else { ?>

<a href="<?php the_permalink(); ?>" title="" rel="bookmark">
<img src="https://www.SITE/IMAGE.jpg" alt="" title="" />
</a>
<?php } ?>

I learned about that here, and it’s marvelous. I have been searching all day, however, on this site and others, for a solution in how to add these thumbnails as image links to the Previous and Next post on a Single post. Inserting php code into the img src breaks everything, and my mind has melted by this point.