Index: wp-content/themes/twentyfifteen/inc/template-tags.php
===================================================================
--- wp-content/themes/twentyfifteen/inc/template-tags.php	(revision 30042)
+++ wp-content/themes/twentyfifteen/inc/template-tags.php	(working copy)
@@ -271,8 +271,10 @@
 
 	<?php else : ?>
 
-	<a class="post-thumbnail" href="<?php the_permalink(); ?>">
-		<?php the_post_thumbnail(); ?>
+	<a class="post-thumbnail" href="<?php the_permalink(); ?>" aria-hidden="true">
+		<?php
+			the_post_thumbnail(); 
+		?>
 	</a>
 
 	<?php endif; // End is_singular()