Random posts from one category?

Hello,

I have some code on a WordPress site and here it is currently:

<?$my = new WP_Query('category_name=header&showposts=1&orderby=date&order=DESC')?>
				<?php if ($my->have_posts()) : ?>
					<?php while ($my->have_posts()) : $my->the_post(); ?>
					<img src="<?php echo get_meta_image_url(get_post_meta(get_the_ID(), "Header Image", true))?>" alt="#" />
					<?endwhile;?>
				<?endif;?>

How would I make it pick 1 random post from that category in the above code?

Thanks.