User blog comment:Lenopow/Portal 2 Easter Egg - Creepy Message/@comment-68.75.83.128-20130623004624

This was left here by Doug Rattman, the only known survivor of when GLaDOS killed all the Aperture workers with neurotoxin. Although never actually seen, he was an important part of the Portal 1 storyline, where he left multiple pictures and messages like these for the main character, Chell, to find.

There is actually many levels in Portal 2 where you can find paintings like this from him, and you don't need cheats for any of them (including this one).