Yup, Go into layout.tpl, and you will see a list of links and expmales of how drop down works and sub links.
The Header Pic is 980 wide and 150 high.
If you want to reduce the size of it, go into sytles.css
find this line and edit the size you want, but the width 980 is the wide of your site and the nav. But height can be still edited.
#topBanner { background: url(images/topbanner.jpg); width: 980px; height: 150px; }
Hope this helps.