mando il codice della mia jsp nel caso possa servire a qualcuno...
ciao

codice:
<%@ page contentType="image/jpeg" import="java.awt.image.*,java.awt.*,com.sun.image.codec.jpeg.*" %>
<%
         int size_v = 50;
         int size_h = 200;

         BufferedImage bfm = new BufferedImage(size_h,size_v,BufferedImage.TYPE_INT_RGB);
         Graphics g = bfm.getGraphics();

         g.setColor(Color.yellow);
         g.fillRect(0,0,size_h,size_v);
         g.setColor(Color.black);         
         g.setFont(new Font("Courier", Font.BOLD, 10));
         g.drawString(java.net.InetAddress.getLocalHost().toString()+" @ "+String.valueOf(System.currentTimeMillis()), 10, 20); 
         
         
         JPEGImageEncoder enc = JPEGCodec.createJPEGEncoder(response.getOutputStream());

         enc.encode(bfm);
         
         response.getOutputStream().close();
         
%>