这里设置的是每隔5s 切换背景图片 一直无限循环下去；
var t = 0;
function setBackground(){
switch (t%4){
case 0 : {
$('body').css("background","url('page_1.jpg') no-repeat center center fixed");
break;
}
case 1 : {
$('body').css("background","url('page_2.jpg') no-repeat center center fixed");
break;
}
case 2 :{
$('body').css("background","url('page_3.jpg') no-repeat center center fixed");
break;
}
case 3 : {
$('body').css("background","url('page_4.jpg') no-repeat center center fixed");
break;
}
}
t = setTimeout(function(){
setBackground();
},5000);
}