function rotateEvery(sec,id)
{
    var Pictures=new Array();

    // Start Images
    // start_firma.png start_unis.png start_students.png
    Pictures[0] = "start_firma.png";
    Pictures[1] = "start_unis.png";
    Pictures[2] = "start_students.png";

    var which = Math.round(Math.random()*(Pictures.length - 1));
    document.getElementById(id).setStyle({backgroundImage:'url(/images/' + which + ')'})

    setTimeout('rotateEvery('+sec+')', sec*9000);
}