hmmmm, I have new idea:

you can use simple JPanel instead of JViewport:
and you can have any shape - not only rectangular!

import javax.swing.*;
import java.awt.*;
import java.awt.image.RGBImageFilter;
import java.awt.image.FilteredImageSource;
import java.awt.geom.AffineTransform;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

/**
* @author Andrei Kouznetsov
*         Date: 27.02.2004
*         Time: 11:29:17
*/
public class ThumbnailPane extends JPanel {

   ActionListener listener;

   Image thumbnail;
   Image thumbnail2; //better quality or color/grascale to draw inside
shape;
   Shape shape, translatedShape;

   private ZoomPane zoomPane;

   public ThumbnailPane(Image thumbnail, Image thumbnail2, Shape shape) {
       this.thumbnail = thumbnail;
       this.thumbnail2 = thumbnail2;
       this.shape = shape;
       zoomPane = new ZoomPane();
       listener = getZoomPane();
       translate(0, 0);
   }

   public Shape getCurrentShape() {
       return translatedShape;
   }

   public Dimension getPreferredSize() {
       return new Dimension(thumbnail.getWidth(null),
thumbnail.getHeight(null));
   }

   //use it to move your "Viewport"
   public void translate(int x, int y) {
       translatedShape = AffineTransform.getTranslateInstance(x,
y).createTransformedShape(shape);
       if (listener != null) {
           listener.actionPerformed(new ActionEvent(this,
ActionEvent.ACTION_PERFORMED, "moved"));
       }
   }

   public void paint(Graphics g) {
       g.drawImage(thumbnail, 0, 0, null);

       //use following if you want to draw thumbnail  inside viewport
       Graphics2D g2d = (Graphics2D) g;
       g2d.clip(translatedShape);
       g.drawImage(thumbnail2, 0, 0, null);
       g2d.clip(null);

       g.setColor(Color.blue);
       g2d.draw(translatedShape);
   }

   public ZoomPane getZoomPane() {
       return zoomPane;
   }

   class ZoomPane extends JPanel implements ActionListener {

       Image currentView;
       Shape shape;

       AffineTransform transform; //zoom factor

       //you have to implement it
       Image getImageTile(Shape shape) {
           return null;
       }

       public void actionPerformed(ActionEvent e) {
           shape = transform.createTransformedShape(translatedShape);
           currentView = getImageTile(shape);
           repaint();
       }

       public Dimension getPreferredSize() {
           if (shape != null) {
               Rectangle r = shape.getBounds();
               return new Dimension(r.width, r.height);
           }
           return super.getPreferredSize();
       }

       public void paintComponent(Graphics g) {
           g.setColor(getBackground());
           Dimension d = getSize();
           g.fillRect(0, 0, d.width, d.height);
           g.drawImage(currentView, 0, 0, this);
       }
   }

   public static void main(String[] args) {
       //create and load images here
       Image thumbnail2 =
Toolkit.getDefaultToolkit().createImage("imagename");
       RGBImageFilter filter = new GrayFilter(false, 50);
       FilteredImageSource source = new
FilteredImageSource(thumbnail2.getSource(), filter);
       Image thumbnail = Toolkit.getDefaultToolkit().createImage(source);
       Shape shape = new Rectangle(0, 0, 100, 100);

       ThumbnailPane tp = new ThumbnailPane(thumbnail, thumbnail2, shape);

       JFrame frame = new JFrame();
       frame.getContentPane().add(tp);
       JDialog dialog = new JDialog(frame, false);
       dialog.getContentPane().add(tp.getZoomPane());
       frame.pack();
       frame.show();
       dialog.pack();
       dialog.show();
   }
}

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