If custom image header does not exist display text header

Try this:

<?php //Custom header
// Check if this is a post or page, if it has a thumbnail, and if it's a big one
if ( is_singular() &&
has_post_thumbnail( $post->ID ) &&
( /* $src, $width, $height */ $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'post-thumbnail' )) &&
$image[1] >= HEADER_IMAGE_WIDTH ) :
// We have a new header image!
echo get_the_post_thumbnail( $post->ID, 'post-thumbnail', array('usemap' => '#Map') );
elseif ( $img_src = get_header_image () ) : ?>
<img src="https://wordpress.stackexchange.com/questions/33814/<?php header_image(); ?>" alt="<?php bloginfo('name'); ?>" usemap="#Map" />
<?php else: ?>
<h1><?php bloginfo('name'); ?></h1>
<?php endif; ?>