The poster is located at the Museum of History's Museum Offices on the topmost floor, over a desk and on the wall just to the left of the top of the staircase to that level (Lincoln's voice is on the next desk). The poster will not spawn until you start the quest.
To easily find this, go to the cell at the rear of the Museum Offices and go up the staircase leading to the highest floor. Directly at the top of that staircase you will see the poster on the wall next to a desk and a filing cabinet that has fallen over, you can't miss it, it's got a yellowish border to it.
- If the picture is found and sold to Leroy Walker for 100 or 200 caps before meeting Caleb Smith, it can be pickpocketed back from Leroy Walker and given to Caleb. It cannot be sold to Hamlin or, more oddly, to Abraham.
- Though it no longer serves any purpose, this item is still found in the game files for Fallout: New Vegas.