I just got done doing this exact thing in a project. The HTML side looked a bit like this:
<a href="[fullsize]" class="gallerypic" title="">
<img src="[thumbnail pic]" height="90" width="140" alt="[Gallery Photo]" class="pic" />
<span class="zoom-icon"><img src="/images/misc/zoom.gif" width="32" height="32" alt="Zoom"></span>
</a>
Then using CSS:
a.gallerypic{
width:140px;
text-decoration:none;
position:relative;
display:block;
border:1px solid #666;
padding:3px;
margin-right:5px;
float:left;
}
a.gallerypic span.zoom-icon{
visibility:hidden;
position:absolute;
left:40%;
top:35%;
filter:alpha(opacity=50);
-moz-opacity:0.5;
-khtml-opacity: 0.5;
opacity: 0.5;
}
a.gallerypic:hover span.zoom-icon{
visibility:visible;
}
I left a lot of the sample in there on the CSS so you can see how I decided to do the style. Note I lowered the opacity so you could see through the magnifying glass.
Hope this helps.
EDIT: To clarify for your example - you could ignore the visibility:hidden; and kill the :hover execution if you wanted, this was just the way I did it.