Set the wallpaper.

def change(img):
    """Set the wallpaper."""
    if not os.path.isfile(img):
        return

    desktop = get_desktop_env()

    if OS == "Darwin":
        set_mac_wallpaper(img)

    elif OS == "Windows":
        set_win_wallpaper(img)

    else:
        set_desktop_wallpaper(desktop, img)

    logging.info("Set the new wallpaper.")