Set navigationbar size according to background image

I have set an backgroundImage for my navigationbar. This works fine. But I would like to have the navigationbar height to be adjusted to the background image. At the moment the width of the background images is also not set according to the screen size.

I tried setting the height of the navigationbar like described here. This shows a bigger navigationbar for like a second but then it shrinks to its default size again.

Does anyone know how to achieve what I want? Here is an example of what I want to achieve: image

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