infatti ho detto che devi adattarlo al tuo contesto
applica una classe alle immagini in questione. esempio:
codice:
<!DOCTYPE html>
<html>
<head>
<style>
img.light {
  opacity: 0.5;
}

img.light:hover {
  opacity: 1.0;
}

</style>
</head>
<body>

<table cellpadding="3">

<tbody>
<tr>
<td align="center" width="25%"><a href="..." target="_blank"> <img class="light" height="215" src="..." width="215" /></a></td>

<td align="center" width="25%"><a href="..." target="_blank"> <img class="light" height="215" src="..." width="215" /></a></td>

<td align="center" width="25%"><a href="..." target="_blank"> <img class="light" height="215" src="..." width="215" /></a></td>

<td align="center" width="25%"><a href="..." target="_blank"> <img class="light" height="215" src="..." width="215" /></a></td>

</tr>
</tbody>
</table>
</body>
</html>