Because they say it better than me:
http://www.china.org.cn/english/government/210587.htm
Man Detained for Damage Chairman Mao Portrait
Beijing police has detained a man who attempted to burn the portrait of late Chinese leader Chairman Mao Zedong that hangs above the gate to the Forbidden City across the street from Tian'anmen Square.
Gu Haiou, 35, a jobless man from Urumqi, capital of Xinjiang Uygur Autonomous Region, hurled a self-made burning object to the portrait around 5:46 PM Saturday, which caused a slight burnt mark in the lower left part of the portrait, said the Beijing Public Security Bureau. […]The Tian'anmen Administration Committee will replace the damaged portrait with a new one of the same version Saturday night, the bureau said. Armed police are guarding the area and visitors are forbidden to enter the Forbidden City. […]
The portrait, six meters high, five meters wide and weighing about two tons, is a reproduction of the fourth edition of Mao's portrait that has hung at the historic spot since the Republic of China was founded in 1949.
The first edition painted in 1949 showed Mao wearing a military uniform and a cap. The second edition, hung between 1950 and 1952, showed Mao in a green suit. The third edition, painted in the 1950s by portraitist Zhang Zhenshi, was put up between 1953 and 1968. (Xinhua News Agency May 13, 2007)