Navigation Bar Buttons

Hi, I’m fairly new to scripting and wonder if it’s possible to create a “4 State” Navigation Bar.
What I would like to do is have the menu bar slide in from the left, then remain static. I have created gifs for the menu buttons to slide in from the left, then the static, over and down images, but I have no idea how to implement them using javascript. At the moment, the up image gif that slides all of the buttons in works fine, the rollover and the down gifs look ok too but I was wondering if there was a way of changing the up image once the original gif has loaded and been shown.
I know it is possible to this using Flash, but I’m trying to avoid using Flash on the website.

Any help would be great, thanks.