Lavender Brown was right outside the portrait of the Fat Lady when the event occured.
Harry Potter, who had been running through Gryffindor Tower, tripped and fell. His head had come crashing right through the Fat Lady's chest. There was the saviour of the wizarding world with his head stuck in the large bosom of the Fat Lady.
Miss Brown had found this to be quite hilarious, so she preceded to let out short machine gun bursts of giddy giggles, whilst the Fat Lady shrieked. Of course, all this noise drew the attention of the remainder of the student body, not to mention the faculty. All of whom immediately arrived on the scene. Even Professor Snape found the situation to be quite amusing.
Eventually, Colin Creevey came along, after many a threat from one very unamused Harry Potter, and snapped a photograph which will appear in tomorrow's Daily Prophet.