img1on=new Image();
img1on.src="images/about_on.gif";
img2on=new Image();
img2on.src="images/serv_on.gif";
img3on=new Image();
img3on.src="images/sserv_on.gif";
img4on=new Image();
img4on.src="images/locate_on.gif";
img5on=new Image();
img5on.src="images/contact_on.gif";

img1off=new Image();
img1off.src="images/about_off.gif";
img2off=new Image();
img2off.src="images/serv_off.gif";
img3off=new Image();
img3off.src="images/sserv_off.gif";
img4off=new Image();
img4off.src="images/locate_off.gif";
img5off=new Image();
img5off.src="images/contact_off.gif";

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