Prueba con esto por si te sirve:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<title>JavaFile</title>
<SCRIPT LANGUAGE="JavaScript">
<!-- Begin
function PrevFoto(img){
foto1= new Image();
foto1.src=(img);
Controlla(img);
}
function Controlla(img){
if((foto1.width!=0)&&(foto1.height!=0)){
viewFoto(img);
}
else{
funzione="Controlla('"+img+"')";
intervallo=setTimeout(funzione,20);
}
}
function viewFoto(img){
largh=foto1.width+20;
altez=foto1.height+20;
stringa="width="+largh+",height="+altez;
finestra=window.open(img,"",stringa);
}
// End -->
</script>
</head>
<body></div>
<a href="javascript
:PrevFoto('http://www.javafile.com/popupwin/freecd.jpg')" border="0">
<img src="http://www.javafile.com/popupwin/freecd.jpg" border="0"></a>
<a href="javascript
:PrevFoto('http://www.javafile.com/popupwin/banner.gif')" border="0">
<img src="http://www.javafile.com/popupwin/banner.gif" border="0"></a><br>
<br>
</body>
</html>