I found some help online which solved my problem:

I discovered that the file I needed to alter was:

Content-single.php

The advice I got:
Have a look at standard.php in includes/post-formats/ and you should find something like <?php the_post_thumbnail(‘thumbnail’); }?> which you can remove.

This was the code I found which seemed right to me:
<?php
if (has_post_thumbnail() ){
echo ‘<div class=”post-thumb”>’;
the_post_thumbnail();
echo ‘</div><br />’;
}
?>

This is the snippet I replaced the removed text with:
<?php
if(!is_single()){
if ( has_post_thumbnail()) { the_post_thumbnail(‘thumbnail’);}
}
?>

This solution worked perfectly!

Is there anything in this code that needs to be altered or changed for the integrity of this theme?