hi

last year I coded a similar client server, though it transmits a screenshot, it can be adapted to transmit a jpeg (BufferedImage) as well.

Here is the code:

first the client:
Code:

/********************************
 *
 *  Client um beim ScrennCaptureServer einem Bild 
 *  anzufordern und direkt via netzwerk auf die 
 *  eigene maschine zu laden.
 *
 *-------------------------------
 *
 *  Autor Martin Fröhlich (2008
 *
 *********************************/

 
import java.io.*;
import java.net.*;

void setup(){
  size(800, 800);
}

void keyPressed() 
{
  if(key == 's' || key == 'S') {
	PImage img = getImageFromServer("maybites.local", 12001, (int)random(800), (int)random(800));
	if(img != null){
	  image(img, 0, 0);
	}
   }
}

void draw(){
  ;
}

PImage getImageFromServer(String name, int port, int w, int h){
  try {

    InetAddress adress = InetAddress.getByName(name);
    println("try contacting server on: " + adress.getHostAddress() + " port: " + port);
    Socket socketConnection = new Socket(adress.getHostAddress(), port);

    println("..success");

    ObjectOutputStream clientOutputStream = new 
	ObjectOutputStream(socketConnection.getOutputStream());

    ObjectInputStream clientInputStream = new 
	ObjectInputStream(socketConnection.getInputStream());

    int[] dim = {w, h};


    println("send request..");
    clientOutputStream.writeObject(dim);
    println("..sent");

    println("waiting for result..");
    int[] pxl = (int[])clientInputStream.readObject();

    PImage img = new PImage(dim[0], dim[1]);
    img.pixels = pxl;
    img.updatePixels();

    println("..received:" + pxl.length);


    clientOutputStream.close();
    clientInputStream.close();
    
    socketConnection.close();

    return img;

  } 
  catch (UnknownHostException e) {
    println("cannot find server: check host name for missspellings"); 
  }    
  catch (Exception e) {
    System.out.println(e); 
  }    
  return null;
}
 

and here the server:

Code:

/********************************
 *
 *  Server um den ScrennCaptureClient mit einem Bild 
 *  zu bedienen 
 *
 *-------------------------------
 *
 *  Autor Martin Fröhlich (2008
 *
 *********************************/

import java.awt.Robot;
import java.awt.Rectangle;
import java.awt.image.BufferedImage;

int LISTENING_PORT = 12001;

Robot myrobie;

void setup() 
{
  myrobie = null;
  try {
    myrobie = new Robot();
  } 
  catch( Exception e1 ) {
    e1.printStackTrace();
  }

  serve();
}

BufferedImage captureImage(int x, int y, int w, int h){
  Rectangle myScreen = new Rectangle(x, y, w, h);
  BufferedImage myImage = myrobie.createScreenCapture(myScreen);
  return myImage;
}  

void serve(){

  try {
    ServerSocket socketConnection = new ServerSocket(LISTENING_PORT);

    while (true) {

	System.out.println("Server Waiting");
	Socket pipe = socketConnection.accept();

	System.out.println("Client Request received");

	ObjectInputStream serverInputStream = new    
	  ObjectInputStream(pipe.getInputStream());

	ObjectOutputStream serverOutputStream = new 
	  ObjectOutputStream(pipe.getOutputStream());

	int[] req = (int[])serverInputStream.readObject();

	System.out.println("Requested Image: " + req[0] + " x " + req[1]);

	System.out.println("Create Screenshot...");

	BufferedImage bimg = captureImage(0, 0, req[0], req[1]);
	PImage img = new PImage(bimg);
	img.loadPixels();
	int[] pixel = img.pixels;

	System.out.println("..done");

	System.out.println("send image..");
	serverOutputStream.writeObject(pixel);
	System.out.println("..done");

	serverInputStream.close();
	serverOutputStream.close();
    } // end of the while loop

  }  
  catch(Exception e) {
    System.out.println(e); 
  }
}
 

here some links:
http://java.sun.com/docs/books/tutorial/networking/sockets/readingWriting.html
http://www.developer.com/design/article.php/10925_3597071_1

it is possible to send complete objects this way (including objects that contain images) but I wasnt able to make that work, so I just send arrays. this has the obvious limitation that the receiver needs to now about the width and the height of the picture.

hope this helps

cheers

martin