document.getElementById style Q

I have this javascript:

if (window.XMLHttpRequest) {
      newLogoAsk = new XMLHttpRequest();
}
function getNewLogo() {
      if (window.XMLHttpRequest) {
      // branch for IE/Windows ActiveX version
      } else if (window.ActiveXObject) {
            newLogoAsk = new ActiveXObject("Microsoft.XMLHTTP");
      }
      newLogoAsk.onreadystatechange = newLogoChange;
      newLogoAsk.open("GET", "/scripts/randomheader.php");
      newLogoAsk.send(null);
}
function newLogoChange() {
      if (newLogoAsk.readyState == 4) {
            var imG = newLogoAsk.responseText;
            document.getElementById("logo").style.background = 'url(../img/headers/'+imG+') no-repeat left top';
      }else{
            document.getElementById("logo").style.background = 'url(../img/header_loading.jpg) no-repeat left top';
      }
}

which applies a new background style to "logo" in this code:

<div id="logo">
      <div id="shadow">
            <div id="nextimg">
                  <a href="javascript://" title="New Header!" onclick="getNewLogo()"><img id="photonext" src="img/header_button.gif" alt="New Header!"/></a>
            </div>
      </div>
</div>

so, everything here works wonderfully, IN FF. NOT in IE. I get the response back from the server, and everything is great, only:

document.getElementById("logo").style.background

doesn't seem to be working in IE!! Is there a solution???