#container { position: relative; float: left; }
#button { position: absolute; }
#button a img {
	position: absolute;
	left: 0;
	top: 0;
	border: 0;
}

#button a img.primary { z-index: 1; }
#button a img.rollover { z-index: -1; }
#button a:hover img.rollover { z-index: 2; }