How can I store it if I don’t have an iPhone?
You can simply create a locked file where you can keep the picture. Yes, it’s that easy and equally safe.
There’s a third option where you can download an app especially made for that, such as Docket VaxYes or Excelsior (however it depends on which state you live
But if digitalization is not your favorite thing in the world, we get it and you can stick to the classic paper card, it’s valid in both forms.