mira tengo este codigo para el css
#Obj0 {
text-align: center;
position: absolute;
top: 50%;
clip: rect(auto,auto,auto,auto);
background-color: #A05A5A;
padding: 0px;
margin: auto;
border: 1px solid #971B1B;
left: 60%;
right: 60%;
bottom: 50%;
height: 100%;
width: 100%;
-moz-opacity:0.75;
opacity:0.75;
filter:alpha(opacity=′75′);
}
y en el html
<div id="Obj0" style="position: absolute; top: -1px; left: 1px;">
<dd>
<table width="100%" border="0" cellspacing="0" cellpadding="0">
<tr>
<td align="center" valign="middle"><a href="#" onclick="document.getElementById('Obj0').style.dis play = 'none'; "><img src="imgs/formulario/no.jpg" width="16" height="17" border="0" /></a></td>
</tr>
<tr>
<td align="center" valign="middle"><img src="imgs/cartel.jpg" width="227" height="340" /></td>
</tr>
<tr>
<td align="center" valign="middle"> </td>
</tr>
</table>
</dd>
</div>