#button {background: url(/images/hidemenu.gif) no-repeat; position: fixed; left: 150px; top: 40%; z-index: 100;}
#button2 {background: url(/images/showmenu.gif) no-repeat; position: fixed; left: 150px; top: 40%; z-index: 1;}
* html #button, * html #button2 {position: absolute;}