It seems to be working fine for me (Chrome, IE8 and Firefox 3. under Vista).
Are you sure the images are having time to load? Do you get the same problem if you resize the images to 100x100 and re-run the script?
Other thoughts
I'm getting an error on line 320 (sub options for nav 6) because you don't have any item on the page with an id of nav6_sub so that may be causing problems for you.
You're duplicating a lot of code on that page (are you copying and pasting or having it created inside a loop?), you should probably look at creating a jQuery plugin or something. I've always found this page to be useful (as well as the jQuery docs of course): http://www.learningjquery.com/2007/10/a-plugin-development-pattern
Also, it's probably not related, but your HTML around the sub navigation could use some cleaning up. Instead of
<ul>
<li><a href="#" id="nav5" onmouseover="dropDown('nav5_sub')"></a></li>
<div class="sub" id="nav5_sub">
<li>Private Client Log In</li>
<li>Student Log In</li>
</div>
</li>
</ul>
You should have something like have:
<ul>
<li>
<a href="#" id="nav5" onmouseover="dropDown('nav5_sub')"></a>
<ul class="sub" id="nav5_sub">
<li>Private Client Log In</li>
<li>Student Log In</li>
</ul>
</li>
</ul>