var Tags=document.getElementById('des_body_l_t_left_2').getElementsByTagName('span'); 
var iSum= document.getElementById('des_body_l_t_left_2').getElementsByTagName('span').length;

for (i=0;i<iSum;i++)
{
    Tags[i].value = i;
	Tags[i].onclick=function(){changeNav(this.value)}; 
}

function changeNav(idVal){
    var i = 0;
    for (i=0;i<iSum;i++)
    {
        document.getElementById("des_span_" + i).style.backgroundImage="url()";
        document.getElementById("des_div_" + i).style.display="none";
    }

    document.getElementById("des_span_" + idVal).style.backgroundImage="url(/chinatravelNet/images/des_main_" + idVal +".jpg)";
    document.getElementById("des_div_" + idVal).style.display="";
}