Decorator design pattern arrangement problem

import java.awt.image.BufferedImage; import java.io.File;

import javax.imageio.ImageIO;

public class DecoratorPattern {

    public static void main(String[] args) throws Exception {
        Image colorImage = new ColorImage("Image.png");

        decorationTest1(colorImage, "DecoratedImage1");
        decorationTest2(colorImage, "DecoratedImage2");
        decorationTest3(colorImage, "DecoratedImage3");
        decorationTest4(colorImage, "DecoratedImage4");
    }

    static void decorationTest1(Image colorImage, String outputFileName) throws Exception {
        BufferedImage image = colorImage.getImage();
        ColorImage.saveToFile(image, outputFileName, "png");

        System.out.println("Image is decorated. Saved as " + outputFileName + ".png");
    }

    static void decorationTest2(Image colorImage, String outputFileName) throws Exception {
        Rectangle rect = new Rectangle(100, 100, 650, 500);
        Image decoratedImage = new ImageGrayScaleDecorator(colorImage, rect);

        BufferedImage image = decoratedImage.getImage();
        ColorImage.saveToFile(image, outputFileName, "png");

        System.out.println("Image is decorated. Saved as " + outputFileName + ".png");
    }

    static void decorationTest3(Image colorImage, String outputFileName) throws Exception {
        int gridSize = 16;
        Rectangle rect = new Rectangle(100, 100, 650, 500);
        Image decoratedImage = new ImageMosaicDecorator(new ImageGrayScaleDecorator(colorImage, rect), gridSize);

        BufferedImage image = decoratedImage.getImage();
        ColorImage.saveToFile(image, outputFileName, "png");

        System.out.println("Image is decorated. Saved as " + outputFileName + ".png");
    }

    static void decorationTest4(Image colorImage, String outputFileName) throws Exception {
        int gridSize = 16;
        Rectangle rect = new Rectangle(100, 100, 650, 750);
        Image decoratedImage = new ImageMosaicDecorator(colorImage, rect, gridSize);

        BufferedImage image = decoratedImage.getImage();
        ColorImage.saveToFile(image, outputFileName, "png");

        System.out.println("Image is decorated. Saved as " + outputFileName + ".png");
    }
}

I am struggling about Decorator design pattern. In this code i take an image and saved it as it is first. Then, i tried to make it grey. After, mozaic on it. For other parts, there's nothing wrong (utilities to change image). But because i am new to that concept, i dont have enough understanding this pattern.

Can you please help me how to do that?

interface Image {
    BufferedImage getImage();
}

class ColorImage implements Image {
    private BufferedImage image;

    ColorImage(int width, int height) {
        image = new BufferedImage(width, height, BufferedImage.TYPE_INT_ARGB);
    }

    ColorImage(String fileName) throws Exception {
        File imageFile = new File(fileName);
        image = ImageIO.read(imageFile);

    }

    static void saveToFile(BufferedImage image, String fileName, String extension) throws Exception {
        File outputfile = new File(fileName + "." + extension);
        ImageIO.write(image, extension, outputfile);
    }

    @Override
    public BufferedImage getImage() {
        return image;

    }
}

class ImageGrayScaleDecorator implements Image {

    public ImageGrayScaleDecorator(Image colorImage, Rectangle rect) {
        image=new BufferedImage(colorImage, rect);
    }

    @Override
    public BufferedImage getImage() {
        BufferedImage rect = null;
        return rect;
    }

}

class ImageMosaicDecorator implements Image {

    public ImageMosaicDecorator(Image colorImage, Rectangle rect, int gridSize) {
    }

    public ImageMosaicDecorator(ImageGrayScaleDecorator imageGrayScaleDecorator, int gridSize) {
    }

    @Override
    public BufferedImage getImage() {
        return null;
    }
}