ramdom bg image cells

I want one class style like .td1 in different cells
And have every cell a different random bg-image
It works with one cell and id
Help please

<script type="text/javascript">
    var images = new Array("px/1.png","px/2.png","px/3.png","px/4.png","px/5.png")
    onload=function() {
    var rand = Math.round(Math.random()*images.length-1)
    td1.style.background="url('"+images[rand]+"')"
    }