img1 = new Image();
img1.src = "file.ext";

img2 = new Image();
img2.src = "file.ext";

img3 = new Image();
img3.src = "file.ext";


img4 = new Image();
img4.src = "file.ext";

function change(num){

document.images["linkpic"].src = "img/menu/" + num + ".jpg";

}
function changeback(){

document.images["linkpic"].src = "img/menu/0.jpg";

}