BufferedImage image = ImageIO.read("filepath");
int startx = 0, starty = 0;
for (int r = 0; r < 100; r++) {
   for (int c = 0; c < 100; c++) {
       image.setRGB(startx + c, starty + r, Color.YELLOW.getRGB());
   }
}

frame.getContentPane().add(new JLabel(new ImageIcon(resize(image, width, height))));

private BufferedImage resize(BufferedImage img, int newW, int newH) {
        Image tmp = img.getScaledInstance(newW, newH, Image.SCALE_SMOOTH);
        BufferedImage dimg = new BufferedImage(newW, newH, BufferedImage.TYPE_INT_ARGB);

        Graphics2D g2d = dimg.createGraphics();
        g2d.drawImage(tmp, 0, 0, null);
        g2d.dispose();

        return dimg;
    }

frame.getContentPane().add(new JLabel(new ImageIcon(image)));