I feel you. I really do.
I understand that your target population (in this case) is college students, who are often far too busy to notice the various posters stuck up on bulletin boards, walls, windows, and assorted other vertical surfaces across campus.
I understand that, as you are not permitted to enter student rooms at whim to leave posters there (like charming advertising elves), you must find other means to promote your event.
I also understand that the interior walls of bathroom stalls are, realistically, among the few more-or-less public spaces in which all students (as well as faculty and staff) are guaranteed to put in appearance, and that their attention in such spaces will be relatively uninterrupted.
I hope, however, that you will understand me when I say to you that being stared at by a series of variously aged and invariably creepy photographs of Justin Bieber does not enhance the waste elimination experience. I suspect it also does not set up a healthy psychological association vis-a-vis your event. You may wish to reconsider such advertisements in the future.