Where do I need to go in Portal 2 to get the Portrait of a Lady achievement? I thought it would've been the portrait of Chell I found (the one that was on the cover of the comic) but that apparently was not it.
As ChrisF stated, it's at the beginning of Chapter 7. When you exit the previous chamber you'll see a door to an office on the left, shoot a portal on to the wall next to the door and one in the hallway up ahead. Go into the office and there's a portrait of Caroline and Cave Johnson in there. Stand in front of it for a second and you should get the achievement.
If written instructions aren't clear you can look at this video: