Add a link to an image in a css style sheet

You can not do that…
via css the URL you put on the background-image is just for the image.
Via HTML you have to add the href for your hyperlink in this way:
Your logo

With text-indent and some other css you can adjust your a element to show just the image and clicking on it you will go to your link.

EDIT:
I’m here again to show you and explain why my solution is much better:
Your logo name

This block of HTML is SEO friendly because you have some text inside your link!
How to style it with css:
#logo {
background-image: url(images/logo.png);
display: block;
margin: 0 auto;
text-indent: -9999px;
width: 981px;
height: 180px;
}

Then if you don’t care about SEO good to choose the other answer.

You don’t add links to style sheets. They are for describing the style of the page. You would change your mark-up or add JavaScript to navigate when the image is clicked.
Based only on your style you would have: