guzzle

$files = [];
$index = 1;
foreach ($pictures as $picture) {
 $filename = 'image_' . $offer_id . '_' . $index . '.jpg';
 $destination = 'sites/default/files/product-photos/' .

use GuzzleHttp\ClientInterface;
/**
 * An http client.
 *
 * @var \GuzzleHttp\ClientInterface
 */
protected $httpClient;
 /**
 * The form constructor.
 *
 * @param \GuzzleHttp\ClientInterface $http_client
 *   Guzzle client interface.
 */
public 

$external_file = 'https://www.example.com/test.png'
$destination = 'sites/default/files/test.png';
$response = \Drupal::httpClient()->get($external_file, ['sink' => $destination]);