Same thumbnail repeating for archives- but links correct!?! Help

Here is the link to the page I am referring to:

http://mcnairevansblog.com/archives

Although the same image repeats, the links are directing to the correct post. How would I display the first image from each corresponding post? My (archives.php)
code is as follows:

<div class="entrycat">
<br /> 

<!-- archives with the_excerpt so the page shows a grid of thumbnails for each post-->
<?php $arc_query = new WP_Query('orderby=post_date&order=DESC&showposts=-1'); ?> <?php while ($arc_query->have_posts()) : $arc_query->the_post(); ?> <a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>"><?php echo get_first_image('WIDTH=75&HEIGHT=75'); ?></a>
<!--<?php the_title(''); ?>-->
<?php endwhile; ?>
</div>

and my (functions.php) code is here:

function get_first_image($post_ID, $fullsize=false, $max_dims=false){
	$thumbargs = array(
	'post_type' => 'attachment',
	'post_status' => null,
	'post_parent' => $post_ID
	);
	$thumbs = get_posts($thumbargs);
	if ($thumbs) {
		$num = count($thumbs)-1;
		return get_attachment_innerHTML($thumbs[$num]->ID, $fullsize, $max_dims);
	}
}

Thanks in advance for any help or advice!