Source
File: wp-includes/customize/class-wp-customize-header-image-setting.php
public function update( $value ) {
global $custom_image_header;
// If _custom_header_background_just_in_time() fails to initialize $custom_image_header when not is_admin().
if ( empty( $custom_image_header ) ) {
require_once ABSPATH . 'wp-admin/includes/class-custom-image-header.php';
$args = get_theme_support( 'custom-header' );
$admin_head_callback = isset( $args[0]['admin-head-callback'] ) ? $args[0]['admin-head-callback'] : null;
$admin_preview_callback = isset( $args[0]['admin-preview-callback'] ) ? $args[0]['admin-preview-callback'] : null;
$custom_image_header = new Custom_Image_Header( $admin_head_callback, $admin_preview_callback );
}
// If the value doesn't exist (removed or random),
// use the header_image value.
if ( ! $value ) {
$value = $this->manager->get_setting( 'header_image' )->post_value();
}
if ( is_array( $value ) && isset( $value['choice'] ) ) {
$custom_image_header->set_header_image( $value['choice'] );
} else {
$custom_image_header->set_header_image( $value );
}
}