var preimg = new Array(2);
function preloadImg(){
for (i=1;i<3;i++){
preimg[i] = new Image;
preimg[i].src = "img/mn_"+(i)+"_on.gif";
}
}

function chimg(over){
img1 = new Image;
img1.src = "img/"+over+"_on.gif";
document.images[over].src = img1.src;
return true;
}

function bkimg(out){
img2 = new Image;
img2.src = "img/"+out+"_off.gif";
document.images[out].src = img2.src;
return true;
}