#logolink
{
display: block;
height: 100px;
width: 200px;
background-image: url(logo.jpg);
}
...
<a id="logolink" href="http://whatever"></a>
The anchor tag is naturally inline so you need to make it a block in order
to hold a shape without inner content. The height & width are those of the
image.
To make this work in a non-CSS user agent as well, you might want to add
something like this:
#bglink span
{
display: none;
}
...
<a id="bglink" href="#">
<span>Click my logo</span>
</a>
If CSS isn't enabled, the text link replace the image link.