These options are usually left blank. However, here is an explanation just so you know what these options are for.
Nav Image (menu image)
Most sites use a regular text menu. In which case you would leave the Nav Image box blank. However, if your site uses an image for your menu options, this is where you would upload that image. An example of a Nav Image can be seen below.
Header Image (page title)
Header Image is for a page title image. Pappa-Pak defaults to a text page title but you can make an image using a fancy font for your page titles. This is where you would upload that image if you were using an image for your page title.
List Image (category image)
If you are using a thumbnail image to represent each of your categories (for those with just a catalog link in the menu and not actual categories showing in the menu), the List Image box is where to upload this particular image. Without an image in this box, the visitor to the website would see just regular text links instead.