Recent Post Thumbnail with Categories

I recently created a new theme and learned how to insert a recent post thumbnail into a template page. It works fine, but I’m wondering if anyone knows how I can alter the code to only show posts from a particular category. This code is in the functions.php file and is as follows:

if ( function_exists( 'add_theme_support' ) ) {
        add_theme_support( 'post-thumbnails' ); } //Adds thumbnails compatibility to the theme
set_post_thumbnail_size( 300, 200); // Sets the Post Main Thumbnails
    add_image_size( 'recent-thumbnails', 375, 250); // Sets Recent Posts Thumbnails

function recentPosts() {
	$rPosts = new WP_Query();
	$rPosts->query('showposts=9');
		while ($rPosts->have_posts()) : $rPosts->the_post(); ?>
			<li>
				<a href="<?php the_permalink();?>"><?php the_post_thumbnail('recent-thumbnails'); ?></a>
			</li>
		<?php endwhile;
	wp_reset_query();
}

And in the template file the function is called with the following:
<?php echo recentPosts(); ?>

Any help would be great!