How do I properly map a `MagImageScalingCallback` using JNA?

The callback is proper – there is no flaw in your code, besides you are using deprecated functions.

Consider using this:

try {
    Rectangle screenRect = new Rectangle(Toolkit.getDefaultToolkit().getScreenSize());
    BufferedImage  capture = new Robot().createScreenCapture(screenRect);
    ImageIO.write(capture, "JPEG", new File("printed1.jpg"));
} catch (Exception e) {
    e.printStackTrace();
}