How to set the_post_thumbnail
so that it doesn't use an array for its size, but instead can be set with a 100% width and auto height:
<?php $ht_featured_img = get_option('ht_featured_img');
if ($ht_featured_img == "true") { ?>
<?php if ( (function_exists('has_post_thumbnail')) && (has_post_thumbnail()) ) { /* if post has a thumbnail */ ?>
<div class="post-image">
<?php the_post_thumbnail( array(1215,9999) ); ?>
</div><!--post-image-->
<?php } ?>
<?php } ?>