They will tell you that you are not allowed to put anything up because it is a fire hazard or something like that. But that does not stop anyone. Me and my roommate had pictures and posters all over our walls, and we even put up a tapestry in our living room. It covered an entire wall. And I know ALL of my roommates had something on their walls.
On my roommate's first program she said her entire apartment took down their stuff for their first inspection, only to find out that no one cared. We were never once cited for stuff on our walls, and I have never heard of anyone else.