The following ought to be much closer to what you want to achieve:
<div class="imgHentry">
<a href="https://wordpress.stackexchange.com/questions/74228/<?php the_permalink(); ?>" class="goto-icon">
<?php the_post_thumbnail(
'thumbnail',
array( 'alt' => get_post_meta( $post->ID, 'alt_tag', true ) )
); ?>
</a>
</div>