There is not depth parameter for get_posts. See http://codex.wordpress.org/Template_Tags/get_posts
You will need to check to see whether the page has a parent and display it if it does not have a parent.
Once you are in the foreach loop add an if statement
<?php if( !$post->parent ): ?>
<li><a href="https://wordpress.stackexchange.com/questions/26866/<?php the_permalink(); ?>" title="<?php the_title(); ?>">
<?php if ( has_post_thumbnail() ) { the_post_thumbnail('portrait');} else { echo '<img src="#" alt="Something Went Wrong?" title="Sorry - Something went wrong?" />';}?></a>
<span><?php meta('issue-number'); ?></span>
</li>
<?php endif: ?>