if (document.images) { 

var img1on = new Image();
img1on.src = "image/btn1b.gif";
var img1off = new Image();
img1off.src = "image/btn1.gif";

var img2on = new Image();
img2on.src = "image/btn2b.gif";
var img2off = new Image();
img2off.src = "image/btn2.gif";

var img3on = new Image();
img3on.src = "image/btn3b.gif";
var img3off = new Image();
img3off.src = "image/btn3.gif";

var img4on = new Image();
img4on.src = "image/btn4b.gif";
var img4off = new Image();
img4off.src = "image/btn4.gif";

} 

function on(name) { 

if (document.images) { 
document.images[name].src = eval(name + 'on.src'); 
} 

} 

function off(name) { 

if (document.images) { 
document.images[name].src = eval(name + 'off.src'); 
} 

}