This is a two-part achievement, as you can only send one photograph at a time.
You can send photos by talking to postmen. The first you come across is essentially story related. In Part I, Chapter 3: The Arrival
, Major Barrett will introduce you to the postman and you will automatically have an option to send a photo back to Julia. Choose any one. You can press
to exit without sending a photo, so don't do that!
Skip forward to Part I, Chapter 4: Above & Below
. As Harry, follow the Major. He will go forwards and then turn right and head into a bunker with a flag in front of it. Just beyond this bunker is the second postman. If Harry is not narrating
, the postman will call you over so he's pretty hard to miss.
Talk to him and choose any photo to send, which will unlock the achievement.
There are some other postmen in the game but this is the earliest opportunity to get this achievement.