var direction = true;
var opacity = 0;

function setAlpha()
{
  if (direction){
    if (opacity < 100){
      opacity += 10;
    }
    else{
      direction = false;
    }
  }
  else{
    if (opacity > 0){
      opacity -= 10;
    }
    else{
      direction = true;
    }
  }
  img1.style.filter = "alpha(opacity=" + opacity + ",enabled=1)";
}