Page Template: Irregular Width Subpage Gallery

Hello all,
Having a small issue that is outside of my realm of expertise, and I’m hoping that someone can help me figure out how to markup what I’m trying to do.

First off, if you check out this site, I’m trying to make a gallery page that operates in almost the exact same fashion.

My website will have a parent page called “My Work” and all its subpages will be hidden from the main navigation. Once you go to “My Work”, I want there to be thumbnails of all the galleries from the various subpages displayed. I sort of know how to do this (here is my code as it stands now):

<div class="subpages">
		<span style="display:none;"><?php the_ID(); ?></span>
		<?php $parent = $post->ID; ?>
		<?php
		query_posts('posts_per_page=15&post_type=page&post_parent='.$parent);
		 while (have_posts()) : the_post();
		?>

		<?php $image_thumb = get_post_meta($post->ID, 'image-thumb', true); ?>
		<div class="thumbnail-1"><a href="<?php the_permalink(); ?>"><?php the_post_thumbnail(); ?></a></div>

		<?php endwhile; ?>
	</div>

So really, I have two problems.

1. I want the thumbnails to have differing widths. So I figure that there need to be four thumbnail divs to account for the two different widths, alternating row-by-row, but I don’t know how to make the loop basically make four individual stops before starting over.

2. The background-image property of each div style will have to be set (inline) via PHP to point to the URL of the image that the subpage has set as its featured image. I don’t even know if this is possible.

I really would appreciate it if any of you guys could lend me a hand!

Best,
Ethan