if(document.images) {
 var imgs = new Array();
 imgs[0] = new Image();
 imgs[0].src = "img/grup.gif";
 imgs[1] = new Image();
 imgs[1].src = "img/grup_o.gif";
 imgs[2] = new Image();
 imgs[2].src = "img/grup_y.gif";
}
function chgImg(id, imgname) {
 if(document.images) {
  if(imgs[imgname]) {
    document.images[id].src = imgs[imgname].src;
  }
  else {
    document.images[id].src = imgname;
  }
 }
}