It's a fading poster of demon from Buffy the Vampire Slayer. It's got a fierce expression and ugly horns, but you recognize it. It's that fear demon from the Halloween Party at the Frat House. You even recall how it ends. The drawing in the book had foreign writing under it, which Giles translated after the demon was destroyed as "Actual Size." Really ugly demon with the power to make your fears real, but also really tiny.
You feel your muscles twitch and contract. You scream with geometrically decreasing volume as suddenly the world seems to move in slow motion. One second you're looking at the poster, the next you're looking at the inside of your shirt as you fall within your now empty clothes as they slowly collapse on the empty air filling them. You land with a kerplop in your briefs as the whole of your empty clothing billows and collapses on you like a circus tent without any support. You struggle and squeak as you work your way out of the heap of clothing. You had no idea how heavy your wallet was, but somehow it ended up on top of your tiny body. It takes forever to get out of the folds of heavy cloth. You finally reach the waist band, when a zoo worker finds your clothing and reports "another streaker" over his radio. He scoops your clothing into a bag, and you're back on the skid mark inside your briefs.
You curse at whoever tossed you in the sack, and then realize you really can...