When I post Picasa slideshows, it interupts the menu so that when you mouse over the navigation, the menu is behind the slideshow (and therefore not visible). I've begun posting the slideshows further down on the pages, but this tends to look awkward.
Do you have a URL of a sample page? If the slideshow outputs a Flash file, then the Flash code that you are embedding on the page by default places the movie above everything else on the page. If that's the case, try adding the following [param] to the Flash code (replacing the brackets for HTML brackets):