Brilliant, I understand that you are going to send me a postcard. Exciting, (at least for me,) isn't it?
Though I personally fancy a maximum card(skip or see Wikipedia if it's new to you), you don't have to. I don't care too much about the picture you choose, although you can have an idea looking at my favourites below.
Save the postcard itself, I would be delighted if you could get a hand-stamped postmark rather than a machine cancelled mark(esp. in Austrlia, Britain, Canada, France, States). Those automated circle postmarks in Deutschland, Nederland and Suomi are fine with me.
Please avoid machine printed value stamp provided you can use traditional ones(esp. in Britain, Deutschland, Belgie & c. ). Combination of stamps welcome, but you'd better not overpaying the postage.
Okay, that's too much to ask, isn't it? Just do whatever you like (meanwhile pretending you didn't see above); after all, it's your call. And you can call me John.
(Side note: I recently discovered Carl Larsson, and love his watercolour works in "Ett Hem". )