Either simply put an image behind the menu on the page itself
Or go to Macintosh HD, Applications, select iWeb, ctrl-click on it, delect Show Package contents, Contents, Resources, Themes, find the Theme you're using, select and ctrl-click on it choose Show package contents, Shared and find the menu graphic, then replace it with a bigger one.
Note: I'm not responsible if you mess up something following the second way.
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