Algo así te sirve?
Código:
<html>
<head>
<title> New Document </title>
<script language="javascript">
function tamanio(imag){
var imagen = new Image();
imagen.src = imag;
alert("ancho " + imagen.width + "\nalto " + imagen.height);
}
</script>
</head>
<body>
<a href="javascript:;" onClick="tamanio('493-minoucoeur.jpg');return false">
Imagen</a>
</body>
</html>
<hr noshade size=1><img src="http://www.gograph.com/Images-8712/ClipArt/cat03.gif" height="50" border=0 align="absmiddle"> <font size="2" face="verdana" color="#000000">
bet[/CODE]