Index: wp-content/themes/twentyfifteen/inc/template-tags.php
===================================================================
--- wp-content/themes/twentyfifteen/inc/template-tags.php	(revision 30013)
+++ wp-content/themes/twentyfifteen/inc/template-tags.php	(working copy)
@@ -272,7 +272,14 @@
 	<?php else : ?>
 
 	<a class="post-thumbnail" href="<?php the_permalink(); ?>">
-		<?php the_post_thumbnail(); ?>
+		<?php
+			$thumbnail_id = get_post_thumbnail_id();
+			$alt = get_post_meta( $thumbnail_id, '_wp_attachment_image_alt', true );
+			$args = array( 
+						'alt' => sprintf( __( '%1$s; Permalink to %2$s', 'twentyfifteen' ), $alt, get_the_title() )
+					);
+			the_post_thumbnail( 'post-thumbnail', $args ); 
+		?>
 	</a>
 
 	<?php endif; // End is_singular()