Themes
A friend of mine once sent me a post card with a picture of the entire planet Earth taken from space. On the back it said, "Wish you were here."
— Steven Wright

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
A friend mails a photo of the whole Earth and writes "Wish you were here." The joke is that we're already here, on the planet pictured, so the greeting is absurd. It turns a standard postcard line into a deadpan bit about the scale of space.
Context reviewed