Technically that's not a postcard. I see a mail with maybe a sort of attachment in postcard form. But then I have to print it to get the physical card befor I can do anything with it. Be it stapling together with the other cards or directly sending it to someone. But why not email it then immediately i received it? Saves me lot of trouble.
I'm pretty sure that "mail" means the post office, not email. So you would receive a physical postcard.
How about a very low end tablet that one would put on his/her wall and that would update itself each week with a different picture?