css:
margin:0px auto; 自动居中
display:block; 块显示，图片自动伸缩
width: 400px;
height: 400px;
透明
示例：
width: 325px; height: 378px; display: block; position: absolute; top: 0px; bottom: 0px; right: 0px; z-index: 100; margin: auto 0px auto 0px; opacity:0.9; -moz-opacity:0.9; -khtml-opacity:0.9; background-image: url(../../../_bg.png); background-position: left center; background-repeat: no-repeat;
