Also, if you want the default blog header image to show up on all blog pages (category, single, archive) you can use something like this instead of the standard call…
<?php if(function_exists('show_media_header')){
if ( is_single() || is_category() || is_archive() ) {
echo '<img src="'.get_bloginfo('url').'/wp-content/header-images/'.get_option('dhnd_homepage').'" />';
} else {
show_media_header();
}
} ?>