How to get child product image thumbnail in sales order view in Back-end ?

my Magento is 2.1.6

  This code working but i need to get order current image not configurable image all the time if i created order from the backend and select the configurable product and choosing another color instead of default.

      $_item = $block->getItem();
    $objectManager = \Magento\Framework\App\ObjectManager::getInstance();
    $product = $objectManager->create('Magento\Catalog\Model\Product')->load($_item->getProduct()->getId());
    $thitem = $_item->getProduct()->getThumbnail();

Also How to get image url by sku if possible?