Twenty Ten child theme – getting post thumbnails working

I’m new to WordPress and am creating my first WP theme as a Twenty Ten child theme. I want to show a post thumbnail on the homepage and on the category pages, and have tried several tutorials and just can’t get them to show up – I must be missing something!

Most recently I tried this tutorial: http://themesforge.com/wordpress/add-new-native-wordpress-post-thumbnails-to-your-theme/, putting the code for index.php in loop.php as I am using version 3 of WordPress.

My child theme’s functions.php looks like this:

<?php
/* switch on support for post thumbnails - WordPress 2.9 onwards */

if ( function_exists( 'add_theme_support' ) ) { // Added in 2.9
 add_theme_support( 'post-thumbnails' );
 set_post_thumbnail_size( 100, 100, true ); // Normal post thumbnails
 add_image_size( 'hp-post-thumbnail', 200, 200, true ); // Homepage thumbnail size
 add_image_size( 'single-post-thumbnail', 300, 9999 ); // Permalink thumbnail size
}
?>

The relevant section of loop.php:

<?php /* How to display all other posts. */ ?>

	<?php else : ?>
		<div id="post-<?php the_ID(); ?>" <?php post_class(); ?>>
			<h2 class="entry-title"><a href="<?php the_permalink(); ?>" title="<?php printf( esc_attr__( 'Permalink to %s', 'twentyten' ), the_title_attribute( 'echo=0' ) ); ?>" rel="bookmark"><?php the_title(); ?></a></h2>

			<div class="entry-meta">
				<?php twentyten_posted_on(); ?>
			</div><!-- .entry-meta -->

	<?php if ( is_archive() || is_search() ) : // Only display excerpts for archives and search. ?>
			<div class="entry-summary">
				<?php the_excerpt(); ?>
			</div><!-- .entry-summary -->
	<?php else : ?>
			<div class="entry-content">
            	<?php if ( function_exists( 'add_theme_support' ) ) : ?>
					  <?php if ( has_post_thumbnail() ) : ?>
                            <div class="postthumb">
                            <?php the_post_thumbnail( 'hp-post-thumbnail' ); ?>
                            </div>
                        <?php endif; ?>
                    <?php endif; ?>
				<?php the_content( __( 'Continue reading <span class="meta-nav">&rarr;</span>', 'twentyten' ) ); ?>
				<?php wp_link_pages( array( 'before' => '<div class="page-link">' . __( 'Pages:', 'twentyten' ), 'after' => '</div>' ) ); ?>
			</div><!-- .entry-content -->
	<?php endif; ?>

Can anyone please tell me what I have done wrong, or perhaps a better way of doing it? Thank you for your time.