Just so we're clear and so you can understand for the future, hosting thorough Posterous doesn't count as having web hosting.
What I was asking was do you have a web hosting account with someone like
GoDaddy, blueHost,DreamHost?
I don't mean to be discouraging, but like esmi pointed out, this won't work.
Looks like Posterous is using a more Tumblr/Python style code as opposed to PHP.
For example:
WordPress:
<?php if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>
<li>
<div class="work-content">
<?php if(has_post_thumbnail() == false){?>
<div class="work-excerpt">
<?php the_excerpt();?>
</div>
<?php }else{?>
<div class="post-thumb">
<?php the_post_thumbnail();?>
</div>
<?php }?>
<div class="work-title">
<h1><a href="<?php the_permalink();?>"><?php the_title();?></a></h1>
</div>
</div>
</li>
<?php endwhile; else: ?>
<p><?php _e('Sorry, no posts matched your criteria.'); ?></p>
<?php endif; ?>
Posterous:
{block:Fan}
<li>
<a href="{FanProfileLink}" rel="nofollow" title="{FanName}'s profile">
<img src="{FanPortraitURL-20}" alt="{FanName}">
</a>
<a href="{FanProfileLink}" rel="nofollow">{FanName}</a>
</li>
{/block:Fan}
Also a theme usually(but not always) spans multiple files and can easily expand beyond the few sections posterous allows you to edit.