io uso questo (trovato sul forum)
nn so se va su mozilla:

<html>
<title>Foto</title>

<script language="JavaScript">
window.focus()
AW=screen.availWidth
AH=screen.availHeight
function Resetta()
{
caricamento.style.display='none'
foto.style.display=''

larg=document.images["<%=Percorso%>"].width+30;
alt=document.images["<%=Percorso%>"].height+35;
y=(AW-larg)/2

if (alt>AH){
newalt=AH
}else{
newalt=alt
}
if (larg>AW){
newlarg=AW
y=0
}else{
newlarg=larg
}
window.resizeTo(newlarg,newalt);
window.moveTo (y,0)

}

</script>

<link rel="stylesheet" href="/CSS/StyleSheet.css" type="text/css">
<body topmargin=0 leftmargin=0 onload="Resetta()">
<div id="caricamento">
<table border=0 width="100%" height="100%">
<tr><td valign="middle" align="center" class="black10Bold">
Caricamento foto in corso...
</table>
</div>
[img]<%=Percorso%>[/img]" id="foto" style="display:none">
</body>
</html>