if (document.images)
 
{
   img1 = new Image();
   img2 = new Image();
   img3 = new Image();
   img4 = new Image();
   img1.src = "picts/but01_on.gif";
   img2.src = "picts/but02_on.gif"
   img3.src = "picts/but03_on.gif";
   img4.src = "picts/but04_on.gif";
}