Hi,
HikaShop is not setting any permission on the files when it save them. It simply uses the imagejpeg / imagegif / imagepng functions of PHP to save the files. So I would assume that these are the default permissions set by your PHP / server based on how the php.ini or your server user access settings are configured.
However, I searched a bit online and couldn't find information on how to change that and the php.net pages for these functions don't mention anything related to file permissions:
www.php.net/manual/en/function.imagejpeg.php