What I did was; using the Taxonomy Images plugin to associate an image with each category, then to my theme’s header.php, below the <a>
line in the above code;
<?php
$image_id = apply_filters( 'taxonomy-images-queried-term-image-id', 0 );
if ( is_category() && ! empty( $image_id )):
print apply_filters( 'taxonomy-images-queried-term-image', '',
array(
'image_size' => 'header'
) );
else : ?>
And add <?php endif; ?>
below the endif ‘end check for featured…’
and defined the header image size in functions.php with
add_image_size( 'header', 960, 260, true );