Get image field default image

$field_config = FieldConfig::loadByName('user', 'user', 'user_picture');
if ($field_config) {
  $default_value = $field_config->getSetting('default_image');
  if (!empty($default_value) && !empty($default_value['uuid'])) {
    $file = \Drupal::entityTypeManager()
      ->getStorage('file')
      ->loadByProperties(['uuid' => $default_value['uuid']]);
    $file = reset($file);
    if ($file) {
      $image_url = $file->createFileUrl();
    }
  }
}