Photograph No.5, found by Sirius Black in search of the Map, shoved down the back of James' trunk.

A figure is running up and down the Slytherin table in the Great Hall at night. If observed closely, you can the stars on the ceiling illuminate his jet-black, messy hair, and his drunk yet cheeky grin plastered on his face.

The picture was blown up, multiplied, and posted all over the school.

This person is wearing a bra.

Only a bra.