png：
http://book.51cto.com/art/200903/112725.htm
bmp：
http://book.51cto.com/art/200903/112712.htm
jpg：
http://wenku.baidu.com/view/bb08bad376eeaeaad1f330f0.html
gif：
http://www.cnblogs.com/thinhunan/archive/2006/04/12/372942.html