In the room that is spinning (with a mannequin sitting at a table and a chair hanging from the ceiling). Once you place the chair in the correct spot a crawl space hole will open up behind the standing mannequin. Duck under it and the poster will be to the left, on the right side of a stage-like area.
The following guide is not mine, credit goes to PS5Trophies, but it shows exactly where to get all of the in-game collectibles, including the one detailed above: