I found the problem HELP/background height

sketchygrid.com
My blue background works in both ie and ff but only with an exact hieght. Is there a way i can get it to exapnd just as much as my post do?

#mainpage {
	background-image: url(https://www.sketchygrid.com/images/bg_1.png);
	margin-left: auto;
	margin-right: auto;
	width: 1025px;
	background-color: #678398;
	border-left: 1px solid #000;
	border-right: 1px solid #000;
	border-color: #000;
	padding-left: 10px;
	padding-right: 10px;
	color: #FFC;
	background-repeat: repeat;
	height: 2000px;
}

<?php get_header(); ?>

<div id="container">
	<div id="mainpage">
		<div class="main2">
			<div class="content">

<div class="topcont3"><h5>Blog</h5></div>

<?php if (have_posts()) : while (have_posts()) : the_post(); ?>

<div class="topcont">

<h2 class="storytitle"><a href="<?php the_permalink() ?>" rel="bookmark"><?php the_title(); ?></a></h2>

</div>

<div class="post" id="post-<?php the_ID(); ?>">

<div class="meta"><?php _e("Filed under:"); ?> <?php the_category(',') ?> — <?php the_tags(__('Tags: '), ', ', ' — '); ?> <?php the_author() ?> @ <?php the_time() ?> <?php the_date('','<em>','</em>'); ?> <?php edit_post_link(__('Edit This')); ?></div>

		<div class="storycontent"><?php the_content(__('(more...)')); ?></div>

<div class="feedback"><?php wp_link_pages(); ?><?php comments_popup_link(__('Comments (0)'), __('Comments (1)'), __('Comments (%)')); ?></div>

<?php comments_template(); ?>

</div>

<?php endwhile; else: ?>

<h3><?php _e('Sorry, no posts matched your criteria.'); ?></h3>

<?php endif; ?>

<div class="topcont">
<?php posts_nav_link(' — ', __('&laquo; Newer Posts'), __('Older Posts &raquo;')); ?>
</div>

<br />

</div>

<?php get_sidebar(); ?>

</div>
</div>

</div>

<?php get_footer(); ?>