var m=9; //随机显示图片的总数量
var n=Math.floor(Math.random()*m+1) 
switch(n)
{
case 1:
document.getElementById('bg').style.background='url(image/background_left1.jpg)';

break;
case 2:
document.getElementById('bg').style.background='url(image/background_left2.jpg)';

break;
case 3:
document.getElementById('bg').style.background='url(image/background_left3.jpg)';

break;
case 4:
document.getElementById('bg').style.background='url(image/background_left4.jpg)';

break;
case 5:
document.getElementById('bg').style.background='url(image/background_left5.jpg)';

break;
case 6:
document.getElementById('bg').style.background='url(image/background_left6.jpg)';

break;
case 7:
document.getElementById('bg').style.background='url(image/background_left7.jpg)';

break;
case 8:
document.getElementById('bg').style.background='url(image/background_left8.jpg)';

break;
case 9:
document.getElementById('bg').style.background='url(image/background_left9.jpg)';

break;
} 
function clickpic(picname){
	
	document.getElementById('bg').style.background='url(image/'+picname+'.jpg)';
	}