var totalNumOfPics=4;

      function get_cookie(cookie_name){
        var results = document.cookie.match ('(^|;) ?' + cookie_name + '=([^;]*)(;|$)' );
        if(results) return results[2];
        else return null;
      }
      function doThis(){
        var which=get_cookie("picnum");
        which = which !=null ? (parseInt(which,10)+1)%totalNumOfPics : 0;
        document.cookie="picnum="+which+"";
        document.getElementById("main").style.backgroundImage="url('image/bg"+which+".jpg')";
      }