I love you guys, but this is probably one topic you should have left to the intern.
Anyway, in Canada there is one company that does the poster sales at all the universities and travels around every fall, so all the students really do have the same posters. The classics, Piccasso, Monet, lots of Bob Marley and Marilyn Monroe, movie posters like Scarface or Trainspotting that signal a certain masculinity. Back when I was in a dorm was pretty ad hoc, not at all Pinterest-y. It probably would have been considered embarrassing to have put too much effort/money into decorating your room. Richer people would be obvious because they’d have better electronics and gaming consoles. Things started getting more individualized once people moved out of residence and into houses/apartments.