you are welcome ;-)
if this is all sorted, please tick the topic off as 'resolved' - thanks ;-)
PS:
I always wondered why HEADER_IMAGE_WIDTH (or $header_image_width resp.) was used twice in that code; now I know ...
just in case somebody else wants to have a portrait-sized header image in (a child theme of) Twenty Eleven:
the universal code - to allow for landscape and portrait images - should be:
(for Twenty Eleven 1.4 which is slightly different to the earlier versions)
// 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 ), array( $header_image_width, max($header_image_width,$header_image_height) ) ) ) &&
$image[1] >= $header_image_width ) :
// Houston, we have a new header image!