body = document.getElementsByTagName("body")[0];
theBG = window.getComputedStyle(body, "background").backgroundImage;
function hidebg() {
  var nowBG = window.getComputedStyle(body, "background").backgroundImage;
  if (nowBG != theBG) {
    body.style.backgroundImage = theBG;
    document.getElementById("hidebg").innerHTML = "hide bg";
  }
  else {
    body.style.backgroundImage = "none";
    document.getElementById("hidebg").innerHTML = "show bg";
  }
}
// these are here so that nothing shows up when JS is disabled
document.writeln('<div style="position:absolute;top:0;right:10em">');
document.writeln('  <a id="hidebg" onclick="hidebg()">hide bg<\/a>');
document.writeln('<\/div>');