wordpress-post-thumbnail-grid-style.css

/* Post Thumbnail Grid */

.gridcontainer h2 a{color: #333; font-size: 13px;}
.gridcontainer .griditemleft{float: left; width: 150px; margin: 0 30px 20px 0;}
.gridcontainer .griditemright{float: left; width: 150px;}
.gridcontainer .postimage{margin: 0 0 5px 0;}
.gridcontainer .postimage-title {text-align: center;}

wordpress-post-thumbnail-grid.php

<?php
/*
Template Name: Thumbnails
*/
?>
<?php get_header(); ?>

<div class="gridcontainer">

<h1 class="entry-title"><?php the_title(); ?></h1>

<?php
query_posts('posts_per_page=1000'); // Can also be limited to a specific category, like query_posts('cat=4&posts_per_page=1000');
$counter = 1; // Start the counter
$grids = 5; // Grids per row
$titlelength = 20; // Length of the post titles shown below the thumbnails

if(have_posts()) : while(have_posts()) : the_post();

// Show all columns except the right hand side column
if($counter != $grids) :
?>
<div class="griditemleft">
	<div class="postimage">
		<a href="<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>"><?php the_post_thumbnail(); ?></a>
	</div><!-- .postimage -->
	<h2 class="postimage-title">
		<a href="<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>">
		<?php if (mb_strlen($post->post_title) > $titlelength)
			{ echo mb_substr(the_title($before = '', $after = '', FALSE), 0, $titlelength) . ' ...'; }
		else { the_title(); } ?>
		</a>
	</h2>
</div><!-- .griditemleft -->
<?php

// Show the right hand side column
elseif($counter == $grids) :
?>
<div class="griditemright">
	<div class="postimage">
		<a href="<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>"><?php the_post_thumbnail(); ?></a>
	</div><!-- .postimage -->
	<h2 class="postimage-title">
		<a href="<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>">
		<?php if (mb_strlen($post->post_title) > $titlelength)
			{ echo mb_substr(the_title($before = '', $after = '', FALSE), 0, $titlelength) . ' ...'; }
		else { the_title(); } ?>
		</a>
	</h2>
</div><!-- .griditemright -->

<div class="clear"></div>
<?php
$counter = 0;
endif;
$counter++;
endwhile;
endif;
?>

</div><!-- .gridcontainer -->

<?php get_footer(); ?>