Code:

public void saveImageToFile(Image img, String fileName) throws IOException {
    BufferedImage bimg = null;
    int w = img.getWidth(null);
    int h = img.getHeight(null);
    int [] pixels = new int[w * h];
    PixelGrabber pg = new PixelGrabber(img,0,0,w,h,pixels,0,w);
    try { 
        pg.grabPixels(); 
    } catch(InterruptedException ie) { 
        ie.printStackTrace();
    }
    
    bimg = new BufferedImage(w,h,BufferedImage.TYPE_INT_RGB);
    bimg.setRGB(0,0,w,h,pixels,0,w);
    
    FileOutputStream fos = new FileOutputStream(fileName);
    JPEGImageEncoder jpeg = JPEGCodec.createJPEGEncoder(fos);
    jpeg.encode(bimg);
    fos.close();
}