<APPLET CODE="Animazione_2.class" WIDTH="200" HEIGHT="200">...</APPLET>

import java.applet.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.Timer;

public class Animazione_2 extends    Applet
                          implements ActionListener
{
   final int ms=100;
   Timer t;

   final int nFrame=12;
   int frame;
   Image iFrame[];

   final int dx=200;
   final int dy=200;
   Graphics gOFF;
   Image iOFF;

   public void init()
   {
      iOFF=createImage(dx, dy);
      gOFF=iOFF.getGraphics();

      iFrame=new Image[nFrame];
      for(int i=0; i<nFrame; i++)
         iFrame[i]=getImage(getCodeBase(), "PT_anim"+i+".gif");
      frame=0;

      t=new Timer(ms, this);
      t.start();
   }

   public void paint(Graphics g)
   {
      gOFF.drawImage(iFrame[frame], 0, 0, this);
      g.drawImage(iOFF, 0, 0, this);
   }

   public void actionPerformed(ActionEvent e)
   {
      frame++;
      if(frame==nFrame)
         frame=0;
      repaint();
   }

   public void update(Graphics g)
   {
      paint(g);
   }
}

1. Image, Graphics, createImage(), getGraphics()

2. update(), paint()