I think the speed is just fine now...
Yes, the script should go between <HEAD> and </HEAD> preferabbly as the first script (before all other Java Scripts). This will get it pre-loading the images right away....
You should not have to change anything else. What you are trying to do is load the navigation images onto the viewers computer before they are called. This way, when the HTML <img> tag calls for the image, the image is already on the viewers local machine and doesn't have to be downloaded first.
The reason I thought this would help is because your Script that controls the header lowering into position prevents the loading of other images until it has completed. Using a script to pre-load the images BEFORE the lowering header script will make them available much sooner.
Mike.