if (document.images) 
{
   i1 = new Image(); i2 = new Image(); i3 = new Image(); 
   i1.src = "images/red-dot.png"; i2.src = "images/no-dot.png"; i3.src = "images/red-back.png";
   }

function swapImage(imgN,imgU){
if(document.images)document.images[imgN].src=imgU;
}