Hi,
There is no option for that.
HikaShop delete the images on purpose to make sure that they don't eat up too much space on your server in the long run.
You can always remove the code:
$class = hikashop_get('class.file');
$class->deleteFiles('product', $elements, $ignoreFile);
$class->deleteFiles('file', $elements, $ignoreFile);
from the file administrator/components/com_hikashop/classes/product.php to avoid the deletion of the images when deleting products via the interface. But I don't recommend that as you'll loose the change each time you update HikaShop.