add_action( 'after_setup_theme', 'add_category_headers');
function add_category_headers () {
if (is_category('15')){
define( 'HEADER_IMAGE', '%s/images/3.jpg' );
} elseif (is_category('1270')){
define( 'HEADER_IMAGE', '%s/images/2.jpg' );
} else {
define( 'HEADER_IMAGE', '%s/images/1.jpg' );
}