Featured Image in Header of Archive Pages in Twenty Eleven

To show the ‘featured image’ of the top post as header image in an archive page in the Twenty Eleven theme, edit header.php in the child theme;

change this section:

<?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 :
// Compatibility with versions of WordPress prior to 3.4.
if ( function_exists( 'get_custom_header' ) ) {
$header_image_width  = get_custom_header()->width;
$header_image_height = get_custom_header()->height;
} else {
$header_image_width  = HEADER_IMAGE_WIDTH;
$header_image_height = HEADER_IMAGE_HEIGHT;
}
?>
<img src="<?php header_image(); ?>" width="<?php echo $header_image_width; ?>" height="<?php echo $header_image_height; ?>" alt="" />
<?php endif; // end check for featured image or standard header ?>

to:

<?php
/*
* The header image.
* Check if this is an archive and if the latest post has a thumbnail, and if it's a big one
* or Check if this is a post or page, if it has a thumbnail, and if it's a big one
*/
if ( is_archive() ) :
the_post();
$archive_thumb = '';
if( 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 ) :
$archive_thumb = get_the_post_thumbnail( $post->ID, 'post-thumbnail' );
endif;
rewind_posts();
endif;
if ( is_archive() && $archive_thumb ) :
echo $archive_thumb; //the featured image of the latest post in the archive
elseif ( 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 :
// Compatibility with versions of WordPress prior to 3.4.
if ( function_exists( 'get_custom_header' ) ) {
$header_image_width  = get_custom_header()->width;
$header_image_height = get_custom_header()->height;
} else {
$header_image_width  = HEADER_IMAGE_WIDTH;
$header_image_height = HEADER_IMAGE_HEIGHT;
}
?>
<img src="<?php header_image(); ?>" width="<?php echo $header_image_width; ?>" height="<?php echo $header_image_height; ?>" alt="" />
<?php endif; // end check for featured image or standard header ?>

Alphabetize Category Posts Using ‘pre_get_posts’ Action

Occasionally you want to have a different sort order in some of the category archives, for example, you want to output the posts of a specific category in alphabetical order.
Instead of having to create a category template for each of those categories and to add a custom query to change the query parameters, WordPress providdes the ‘pre_get_posts’ action where you can add a corresponding function:

function category_archive_sort_alphabetical( $query ) {
    if ( is_admin() || ! $query->is_main_query() )
        return;

    if ( is_category( array( 'Category One', 'category-two', 'Another Cat' ) ) ) {
		$query->set( 'orderby', 'title' );
		$query->set( 'order', 'ASC' );
        return;
    }
}
add_action( 'pre_get_posts', 'category_archive_sort_alphabetical', 1 );

resources:
http://codex.wordpress.org/Plugin_API/Action_Reference/pre_get_posts
http://codex.wordpress.org/Function_Reference/is_category