Well, at least I was not terribly wrong with my assumptions ??
There is a child theme of a child theme of Twenty Eleven. The code can be found in the child theme of Twenty Eleven – Twenty Eleven/hvc-theme
in header.php:
<?php
// Check to see if the header image has been removed
$header_image = get_header_image();
if ( ! empty( $header_image ) ) :
?>
<?php
// The header image
// 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, HEADER_IMAGE_WIDTH ) ) ) &&
$image[1] >= HEADER_IMAGE_WIDTH ) :
// Houston, we have a new header image!
echo get_the_post_thumbnail( $post->ID, 'post-thumbnail' );
else : ?>
<div id="header-img1" class="slide" style="width: 302px;">
<img src="<?php header_image(); ?>" width="<?php echo HEADER_IMAGE_WIDTH; ?>" height="<?php echo HEADER_IMAGE_HEIGHT; ?>" alt="" style="margin-left: -0px;" style="" /><div class="header-img-info"><p>Historisch Museum<br /><a href="/historisch-museum/">(meer informatie)</a></p></div>
</div>
<div id="header-img2" class="slide" style="width: 147px;">
<img src="<?php header_image(); ?>" width="<?php echo HEADER_IMAGE_WIDTH; ?>" height="<?php echo HEADER_IMAGE_HEIGHT; ?>" alt="" style="margin-left: -302px;" /><div class="header-img-info"><p>Dief- en Duifhuisje<br /><a href="/dief-en-duifhuisje/">(meer informatie)</a></p></div>
</div>
<div id="header-img3" class="slide" style="width: 306px;">
<img src="<?php header_image(); ?>" width="<?php echo HEADER_IMAGE_WIDTH; ?>" height="<?php echo HEADER_IMAGE_HEIGHT; ?>" alt="" style="margin-left: -449px;" /><div class="header-img-info"><p>Begraafplaats<br /><a href="/oude-begraafplaats/">(meer informatie)</a></p></div>
</div>
<div id="header-img4" class="slide" style="width: 245px;">
<img src="<?php header_image(); ?>" width="<?php echo HEADER_IMAGE_WIDTH; ?>" height="<?php echo HEADER_IMAGE_HEIGHT; ?>" alt="" style="margin-left: -755px;" /><div class="header-img-info"><p>Van Cappellenhuis<br /><a href="/regentenkamer/">(meer informatie)</a></p></div>
</div>
<?php endif; // end check for featured image or standard header ?>
<?php endif; // end check for removed header image ?>
It’s quite hacky – and the comments are fun ??
Hope this helps?