I update to make it more complete. Instead of drawing the fillOval on g2 just draw your whiteboard.

import java.awt.image.*;
import java.io.*;
import javax.imageio.*;
import javax.swing.*;
import java.awt.*;
public class Ship extends JPanel{

public void paint(Graphics g){
BufferedImage bi=new java.awt.image.BufferedImage(200, 200, Transparency.BITMASK);
g.fillOval(10,10,50,50);
Graphics2D g2=bi.createGraphics();
g2.setColor(Color.black);
g2.fillOval(10, 10, 50, 50);
try {
File file = new File("newimage.png");
ImageIO.write(bi, "png", file);
System.out.println("Saved");

} catch (Exception ex) {
System.out.println("didnt save");
}
}
public static void main(String[] args){

JPanel jp=new Ship();
JFrame jf=new JFrame();
jf.setPreferredSize(new Dimension(25,100));
jf.add(jp);
jf.pack();
jf.setVisible(true);

}
}