sclass DesktopPaneWithSkyPicture_autoUnload extends JDesktopPane {
  S imageID;
  BufferedImage image;
  new RestartableCountdown countdown;
  double autoUnloadAfter = 60.0;
  
  *(S *imageID) {}
  
  protected void paintComponent(Graphics g) {  
    paintSkyBackground(this, g, _getImage());
  }
  
  BufferedImage _getImage() {
    ret swing(func -> BufferedImage {
      if (image == null)
        image = loadImage2(imageID);
      countdown.start(autoUnloadAfter, r-awt { image = null; });
      ret image;
    }
  }
}

static JDesktopPane jDesktopPaneWithSkyPicture_autoUnload(fS imageID, Color background) {
  ret setBackground(background, swingNu(DesktopPaneWithSkyPicture, loadImage2(imageID)));
}