local funciona, no server naum

Pessoal, tenho o codigo abaixo, que funfa legal aqui na minha maquina. Coloco no servidor pra testar via web e naum dá certo. O que seria?!

<script type="text/javascript">
<!--

function preview(img) {
var img = "file:///"+img;
document.getElementById("preImg").innerHTML = '<img src="'+img+'" alt="" width="101" height="101"" />';

</script>
</head>
<body>
<form id="upImg" action="" method="POST" enctype='multipart/form-data'>
 <table width="100%" border="0" cellspacing="0" cellpadding="0">
	<tr>
 <td width="110" id="preImg"><img src="../Editor/img/sem_foto.jpg" alt="" width="101" height="101" /></td>
 <td><label>
 <input type='hidden' name='MAX_FILE_SIZE' value='50000' />
 <input type="file" id="a_img" onchange="preview(this.value)" />
 </label>
 </td>
	</tr>
 </table>
</form>