While looking at the same problem, I found an example
<style type="text/css">
#topright {
position: absolute;
right: 0;
top: 0;
display: block;
height: 125px;
width: 125px;
background: url(TRbanner.gif) no-repeat;
text-indent: -999em;
text-decoration: none;
}
</style>
<a id="topright" href="#" title="TopRight">Top Right Link Text</a>
The trick here is to create a small, (I used GIMP) a PNG (or GIF) that has a transparent background, (and then just delete the opposite bottom corner.)