Very simple to get.
Near the beginning of chapter 7 (The Reunion), you will come across the first Propulsion Gel (orange gel) puzzle. After you do this one and go through the exit, you will see a gate and a set of stairs to your left. You can portal into this area using a wall at the top of the stairs and a wall by the end of the hallway. After you go through the portal, you will be right next to a small office with a portrait of Cave Johnson and Caroline in it. Just look at the painting. GLaDOS will make a comment and the achievement will pop.