Can be found on both The Storm and Floodgate, I'd recommend Floodgate as it's at the beginning of the level.
"As you make your way down the hill and back into the factory complex, a Marine should radio in, "All squads, report!" At this point, go straight until you reach the corner, where flames can be seen behind a Fence Wall. Here, turn left. As with before, you should see a narrow hallway. The poster is lying on the floor in one of the openings along the right wall, past the dead Brute Chieftain with a Gravity Hammer."
It's the first 'hole in the wall' on your right and all you need to do is look at the poster for the achievement to unlock.
Credit goes to http://halo.wikia.com/
- this really helped me :)
- credit goes to Halo Completionist