The poster is found in the Ag Center. To get there, you go south from the Ranger Citadel (starting area) and it is the first place you reach.
When inside the center, there are 3 routes you can go. For the poster, you should go left from the initial main room, follow that room to the end and turn right. There is a door here. To the right of this door is the poster. You can press Triangle on it to examine and get the trophy.