Add an image to the subfigure.

        Args
        ----
        filename: str
            Filename of the image.
        width: str
            Width of the image in LaTeX terms.
        placement: str
            Placement of the figure, `None` is also accepted.

def add_image(self, filename, *, width=NoEscape(r'\linewidth'),
                  placement=None):
        """Add an image to the subfigure.

        Args
        ----
        filename: str
            Filename of the image.
        width: str
            Width of the image in LaTeX terms.
        placement: str
            Placement of the figure, `None` is also accepted.
        """

        super().add_image(filename, width=width, placement=placement)