var bImage = new Array();
bImage[2] = "";
bImage[0] = "url(images/olavin_sketch.jpg)";
bImage[1] = "url(images/lh_sketch.jpg)";

function toggleBgImage(id){
     if(document.getElementById(id).style.backgroundImage == bImage[0]){
       document.getElementById(id).style.backgroundImage = bImage[1];
     }else{
       document.getElementById(id).style.backgroundImage = bImage[0];
     }
   }