How to use CSS to switch custom images in Unordered List.

Hi,
I have been playing round with an unordered list that I have used CSS to alter the bullet image from the usual black spot to a ball graphic.  

What I want to do is make it that when you hover over each item in the list which is being used as a "nav" block for my page, the image toggles to a version that looks highlighted (ie slightly brighter colour) to enhance the selection.  In the past I have used JavaScript to switch the images but I am sure you ought to be able to do it with CSS.

I have used the background-image rather than list-style-iamge as another issue I was getting was that the text of the list was not centred vertically on the bullet symbol and it didn't work either as far as the image toggling goes.

I have played around with the background-position attribute and found that when the mouse hovers over the link the background image appears but only in the section of the li element where the text is, not over the existing sphere image.  I then tried making its value negative and as you get nearer to the left edge of the screen it just disappears behind the original image that was layed out in the CSS for the li element.

If you visit this page you will see how it is with a background-position set to -20 so the image is appearing but when I set it to -40 it does not replace the original image.

Lnk to my test page

The Style is in the sheet so if you view the source you should see where I have got to.  Any help with this using CSS would be greatfully accepted.

Siv