if (document.images) {
        
  b1off = new Image;  b1off.src = "images/ssoff.jpg";
  b1on  = new Image;  b1on.src  = "images/sson.jpg";
  b2off = new Image;  b2off.src = "images/cioff.jpg";
  b2on  = new Image;  b2on.src  = "images/cion.jpg";
  b3off = new Image;  b3off.src = "images/tioff.jpg";
  b3on  = new Image;  b3on.src  = "images/tion.jpg";
}

function btnon(imgName) {
  if (document.images) 
    document[imgName].src = eval(imgName + 'on.src')
}

function btnoff(imgName) {
  if (document.images)
    document[imgName].src = eval(imgName + 'off.src')
}