Source

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

public function show_header_selector( $type = 'default' ) {
		if ( 'default' === $type ) {
			$headers = $this->default_headers;
		} else {
			$headers = get_uploaded_header_images();
			$type    = 'uploaded';
		}

		if ( 1 < count( $headers ) ) {
			echo '<div class="random-header">';
			echo '<label><input name="default-header" type="radio" value="random-' . $type . '-image"' . checked( is_random_header_image( $type ), true, false ) . ' />';
			_e( '<strong>Random:</strong> Show a different image on each page.' );
			echo '</label>';
			echo '</div>';
		}

		echo '<div class="available-headers">';
		foreach ( $headers as $header_key => $header ) {
			$header_thumbnail = $header['thumbnail_url'];
			$header_url       = $header['url'];
			$header_alt_text  = empty( $header['alt_text'] ) ? '' : $header['alt_text'];
			echo '<div class="default-header">';
			echo '<label><input name="default-header" type="radio" value="' . esc_attr( $header_key ) . '" ' . checked( $header_url, get_theme_mod( 'header_image' ), false ) . ' />';
			$width = '';
			if ( ! empty( $header['attachment_id'] ) ) {
				$width = ' width="230"';
			}
			echo '<img src="' . set_url_scheme( $header_thumbnail ) . '" alt="' . esc_attr( $header_alt_text ) . '"' . $width . ' /></label>';
			echo '</div>';
		}
		echo '<div class="clear"></div></div>';
	}