Current Menu Item CSS

This has been gone over hundreds of times but I just cannot find a good answer for use in WordPress with IMAGES for your navigation items.

Here’s my markup. First, my navigation menu found in my header.php file.

<div id="nav">
            <ul>
            <li class="hnav""><a href="/">Home</a></li>
            <li class="nnav"><a href="/news">News</a></li>
            <li class="wnav"><a href="/weather">Weather</a></li>
            </ul>
 </div>

Now, the CSS for it:
[CSS moderated as per the Forum Rules. Please just post a link to your site.]

Obviously the li tags already have unique ids. I cannot figure out how to do this anywhere since obviously every page calls the same nav in the header.

Please help!