Something like (in your single.php)
<?php while (have_posts()): the_post(); ?>
<article id="post-<?php the_ID(); ?>" <?php post_class(); ?>>
<time class="post-time" datetime="<?php the_time('Y-m-d'); ?>"><?php the_time('F j, Y'); ?></time>
<?php if (has_post_thumbnail()): ?>
<?php $thumbnail_image = get_posts(
array(
'p' => get_post_thumbnail_id($post->ID),
'post_type' => 'attachment',
)
);
?>
<p class="thumbnail-caption"> <?php echo $thumbnail_image[0]->post_excerpt; ?></p>
<?php the_post_thumbnail('full'); ?>
<?php endif; ?>
<header>
<h1 class="post-title"><?php the_title(); ?></h1>
</header>
<?php the_content(); ?>
<footer>
<p class="post-tags"><?php the_tags(); ?></p>
</footer>
</article>
<?php endwhile; ?>
I haven’t tested the image caption bit: let me know if it doesn’t work, or if you don’t dig the markup.