img1on=new Image();
img1on.src="../../images/m11.jpg";
img2on=new Image();
img2on.src="../../images/m21.jpg";
img3on=new Image();
img3on.src="../../images/m31.jpg";
img4on=new Image();
img4on.src="../../images/m41.jpg";

img1off=new Image();
img1off.src="../../images/m1.jpg";
img2off=new Image();
img2off.src="../../images/m2.jpg";
img3off=new Image();
img3off.src="../../images/m3.jpg";
img4off=new Image();
img4off.src="../../images/m4.jpg";

function imgOn(whichButton) {
  document.images[whichButton].src=eval(whichButton+"on.src");
  }
function imgOff(whichButton) {
  document.images[whichButton].src=eval(whichButton+"off.src");
  }