Js Works On One Server, Not Another

I have a basic js that is used to change some images when the user clicks on a link. it worked fine on my server (IIS 4), but when I upload it and the images to another IIS server the images don't display most of the time. I know this is client side -- so why is the server making a difference??

<script language="javascript" type="text/javascript">

var p = new Array(2);
var t = new Array(2);

p[1] = new Image();
p[1].src = "images/8x8_grid.gif";
t[1] = new Image();
t[1].src = "images/grid.gif";

p[2] = new Image();
p[2].src = "images/3x3_hori.gif";
t[2] = new Image();
t[2].src = "images/hori.gif";

function change_image(whichone) {
thumb.src = t[whichone].src;
pattern.src = p[whichone].src;
maintable.background = pattern.src
}

</script>

Here is how I call change_image...

<A href='javascript:void(0);' onmousedown="change_image(&#391;');return false"><img src="images/btn_13x13.gif" width=13 height=13 border=0></A>