个人还是推荐使用css方式

#bg{
    background-image: url(bg.jpg);
    background-position: center 0;
     background-repeat: no-repeat;
     background-attachment:fixed;
     background-size: cover;
     -webkit-background-size: cover;/* 兼容Webkit内核浏览器如Chrome和Safari */
     -o-background-size: cover;/* 兼容Opera */
     zoom: 1;    
    /* background-repeat: repeat-x; */
    /* background-color: aquamarine; */
}

下面是img方式,
HTML代码

<div id="con"><img id="pic" src="maskimg/star.jpg"></div>

CS代码

 body{ margin:0; padding:0;}
 #pic{ width:100%;}