Where is the Portrait of a Lady achievement?

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.

Solution 1:

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:

Good Luck!