def setImage(self, filename=None) :
        """Defines the image on a BitmapButton.

           PROBLEM : once set with a real image, if the
           PROBLEM : method is called again with an empty
           PROBLEM : or None filename value to reset the
           PROBLEM : BitmapButton, this doesn't work : the
           PROBLEM : previous image is still there.
           PROBLEM :
           PROBLEM : I'm using wx v2.6.3.2.1.5 under Debian Sid.
        """
        if filename :
            image = wx.Image(filename)
            maskcolour = image.GetOrFindMaskColour()
        else :
            image = wx.NullImage
            maskcolour = wx.NullColour

        image.Rescale(176, 144)
        bitmap = wx.BitmapFromImage(image)
        mask = wx.Mask(bitmap, maskcolour)
        bitmap.SetMask(mask)
        self.bitmap_image.SetBitmapLabel(bitmap)