Index: wp-content/themes/twentyeleven/header.php
===================================================================
--- wp-content/themes/twentyeleven/header.php	(revision 22166)
+++ wp-content/themes/twentyeleven/header.php	(working copy)
@@ -93,7 +93,7 @@
 					// 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 ) ) ) &&
+							( /* $src, $width, $height */ $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'post-thumbnail' ) ) &&
 							$image[1] >= $header_image_width ) :
 						// Houston, we have a new header image!
 						echo get_the_post_thumbnail( $post->ID, 'post-thumbnail' );