tegn ny linje igennem mouselistener

class DrawablePicture extends JPanel implements MouseMotionListener {
    private final static String STARTPIC_URL = "http://localhost/bg.jpg";
    private final static int W = 750;
    private final static int H = 250;
    private BufferedImage img;
    private Color drawcol;
    private int lastx;
    private int lasty;
    public DrawablePicture() {
        try {
            img = ImageIO.read(new URL(STARTPIC_URL));
            setPreferredSize(new Dimension(W, H));
            this.addMouseMotionListener(this);
            drawcol = Color.BLACK;
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawImage(img, 0, 0, this);
    }
    public BufferedImage getImg() {
        return img;
    }
    public void mouseDragged(MouseEvent ev) {
        Graphics g = img.getGraphics();
        g.setColor(drawcol);
        g.drawLine(lastx, lasty, ev.getX(), ev.getY());
        repaint();
        lastx = ev.getX();
        lasty = ev.getY();
    }
    public void mouseMoved(MouseEvent ev) {
        lastx = ev.getX();
        lasty = ev.getY();
    }
    public void setColor(Color drawcol) {
        this.drawcol = drawcol;
    }
}