iOS Question B4xDrawer change Bitmap

Hi,
I use B4Xdrawer and I arrive to change the color of BarButton with this code

Dim nx As NativeObject = NavControl
Dim no As NativeObject =App
nx.GetField("navigationBar").RunMethod("setTintColor:", Array(no.ColorToUIColor(c)))

I would like to change bitmap of the button (on for open, on for close) but I do not arrive to do this.
(initial load of bitmap is Ok)
Is it possible?

Thank