A CSS question please

If you would be so kind as to visit www.footbagfreaks.com, you will see a sample site using css. On the main navigation, I am curious about something. What establishes the height of each nav element? They are all uniform. What I would like to do is this, which I hope I can explain. I would like to have a set dimension for each nav element, lets us just say each to be 30px high, by 150px wide. I would like to specify the area, 30 x 150 have a rectangular trans. gif as its background and the initial color of the bg would be the same as the trans. gif. On a mouseover, the bg color would change, and thus would show the the new color through the portion of the trans gif which is transparent. Basically, the same effect as on the logo on the top left of the page. If someone can help me with this, I'd appreciate it so much.