Refer to the links I attached in my previous reply. For example, using:
will prevent the background image from repeating, both horizontally and vertically. But you'll probably see some white area because the image won't fill the entire height of the navigation.
There's a nice article about achieving a way to make your background image "fill" at css-tricks: https://css-tricks.com/perfect-full-page-background-image/ - should be applicable to other elements, including the navigation bar, despite of the fact that the article talked more about background image for a whole page.
Or you can just set the size of the background to fit the navigation bar's height. Whichever you prefer.