Why is paint(Graphics g) not drawing image?

public class Main extends Applet implements Runnable, KeyListener{


private URL base;
private Image image, image1,image2, image3, background;
private Graphics second;
String imageString;

@Override
public void init() {

    // Setting Size of screen
    setSize(800, 600);

    setBackground(Color.BLACK);
    // Set game focus
    setFocusable(true);

    addKeyListener(this);

    // Import frames
    Frame frame = (Frame) this.getParent().getParent();
    frame.setTitle("show image");

    try {
        base = getDocumentBase();
    } catch (Exception e) {
        // TODO: handle exception
    }

    // Image Setups

    // Background
    background = getImage(base, "data/background.png");
    //Images
    image1 = getImage(base, "data/image1.png");
    image2 = getImage(base, "data/image2.png");
    image3 = getImage(base, "data/image3.png");


}

@Override
public void start() {


    Thread thread = new Thread(this);
    thread.start();
}

@Override
public void run() {
    // TODO Auto-generated method stub
    imageString = randomString();
    repaint();

    try {
        Thread.sleep(17);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}

@Override
public void update(Graphics g) {
    if (image == null) {
        image = createImage(this.getWidth(), this.getHeight());
        second = image.getGraphics();
    }

    second.setColor(getBackground());
    second.fillRect(0, 0, getWidth(), getHeight());
    second.setColor(getForeground());
    paint(second);
    paint(g);

    g.drawImage(image, 0, 0, this);


}

public void paint(Graphics g) {

        g.drawImage(background, 0, 0, this);
        if (imageString == "color of green"){
            g.drawImage(image1, 0, 0, this);
        } else if (imageString == "red of color"){
            g.drawImage(image2, 0, 0, this);
        } else if (imageString == null){
            g.drawImage(image3, 0, 0, this);
        }
}



@Override
public void keyReleased(KeyEvent arg0) {
    // TODO Auto-generated method stub

}

@Override
public void keyTyped(KeyEvent arg0) {
    // TODO Auto-generated method stub

}

@Override
public void keyPressed(KeyEvent e) {

    }

public String randomString(){
    int randNum;
    String randString = null;
    String randString1 = null;

    randNum = (int)(Math.random()*2 + 1);
    if (randNum == 1){
        randString = "color";
        randString1 = "green";
    } else if (randNum == 2){
        randString = "red";
        randString1 = "color";
    }

    return randString + " of " + randString1;


}

}