Source

File: wp-admin/includes/class-custom-image-header.php

final public function set_header_image( $choice ) {
		if ( is_array( $choice ) || is_object( $choice ) ) {
			$choice = (array) $choice;
			if ( ! isset( $choice['attachment_id'] ) || ! isset( $choice['url'] ) ) {
				return;
			}

			$choice['url'] = esc_url_raw( $choice['url'] );

			$header_image_data = (object) array(
				'attachment_id' => $choice['attachment_id'],
				'url'           => $choice['url'],
				'thumbnail_url' => $choice['url'],
				'height'        => $choice['height'],
				'width'         => $choice['width'],
			);

			update_post_meta( $choice['attachment_id'], '_wp_attachment_is_custom_header', get_stylesheet() );
			set_theme_mod( 'header_image', $choice['url'] );
			set_theme_mod( 'header_image_data', $header_image_data );
			return;
		}

		if ( in_array( $choice, array( 'remove-header', 'random-default-image', 'random-uploaded-image' ), true ) ) {
			set_theme_mod( 'header_image', $choice );
			remove_theme_mod( 'header_image_data' );
			return;
		}

		$uploaded = get_uploaded_header_images();
		if ( $uploaded && isset( $uploaded[ $choice ] ) ) {
			$header_image_data = $uploaded[ $choice ];

		} else {
			$this->process_default_headers();
			if ( isset( $this->default_headers[ $choice ] ) ) {
				$header_image_data = $this->default_headers[ $choice ];
			} else {
				return;
			}
		}

		set_theme_mod( 'header_image', esc_url_raw( $header_image_data['url'] ) );
		set_theme_mod( 'header_image_data', $header_image_data );
	}