Source

File: wp-includes/theme.php

function get_uploaded_header_images() {
	$header_images = array();

	// @todo Caching.
	$headers = get_posts(
		array(
			'post_type'  => 'attachment',
			'meta_key'   => '_wp_attachment_is_custom_header',
			'meta_value' => get_option( 'stylesheet' ),
			'orderby'    => 'none',
			'nopaging'   => true,
		)
	);

	if ( empty( $headers ) ) {
		return array();
	}

	foreach ( (array) $headers as $header ) {
		$url          = esc_url_raw( wp_get_attachment_url( $header->ID ) );
		$header_data  = wp_get_attachment_metadata( $header->ID );
		$header_index = $header->ID;

		$header_images[ $header_index ]                      = array();
		$header_images[ $header_index ]['attachment_id']     = $header->ID;
		$header_images[ $header_index ]['url']               = $url;
		$header_images[ $header_index ]['thumbnail_url']     = $url;
		$header_images[ $header_index ]['alt_text']          = get_post_meta( $header->ID, '_wp_attachment_image_alt', true );
		$header_images[ $header_index ]['attachment_parent'] = isset( $header_data['attachment_parent'] ) ? $header_data['attachment_parent'] : '';

		if ( isset( $header_data['width'] ) ) {
			$header_images[ $header_index ]['width'] = $header_data['width'];
		}
		if ( isset( $header_data['height'] ) ) {
			$header_images[ $header_index ]['height'] = $header_data['height'];
		}
	}

	return $header_images;
}