Native width of this image, calculated from its width in pixels and
        horizontal dots per inch (dpi).

def default_cx(self):
        """
        Native width of this image, calculated from its width in pixels and
        horizontal dots per inch (dpi).
        """
        px_width = self.image.px_width
        horz_dpi = self.image.horz_dpi
        width_in_inches = px_width / horz_dpi
        return Inches(width_in_inches)