As you've said, it's easy to have it save the image to a file.
The real trick here is having Processing tell your computer to print that file.
This will require a system call.
You'll need to figure out what the command line command for printing a document on your system is.
Then you can probably do it with a call to open().

Thank you. I'm on mac osx, but i don't know very well operations from command line. Someone have some suggestion about it?

Hey Secondsky,

This is how I did it in the past (not tested with Processing 2.x)

1. [--- printertest.pde ---]
2. Printer myPrinter;
3. PImage myImage;
4. PFont myFont;
5. void setup()
6. {
7.   size(200,400);
8.   myPrinter = new Printer();
9.   myImage = loadImage("arch.jpg");
10.   myFont = loadFont("AbadiMT-CondensedLight-24.vlw");
11.   textFont(myFont);
12. }
13. void draw()
14. {
15.   background(255);
16.   image(myImage,0,0,width,width);
17. }
18. void keyPressed()
19. {
20.   if(key=='p' || key=='P') myPrinter.StartPrinting();
21. }
22. [--- printerclass.pde ---]
23. import java.awt.Color;
24. import java.awt.Graphics;
25. import java.awt.Graphics2D;
26. import java.awt.geom.Line2D;
27. import java.awt.print.PageFormat;
28. import java.awt.print.Printable;
29. import java.awt.print.PrinterJob;
30. public class Printer implements Printable {
31.   //--- Private instances declarations
32.   private final double INCH = 72;
33.   public PrinterJob printJob;
34.   /**
35.    * Constructor
36.    */
37.   public Printer() {
38.     //--- Create a printerJob object
39.     printJob = PrinterJob.getPrinterJob();
40.     //--- Set the printable class to this one since we
41.     //--- are implementing the Printable interface
42.     printJob.setPrintable(this);
43.   }
44.   public void StartPrinting()
45.   {
46.     //--- Show a print dialog to the user. If the user
47.     //--- click the print button, then print otherwise
48.     //--- cancel the print job
49.     // this.g2d = myImage;
50.     if (printJob.printDialog()) {
51.       try {
52.         printJob.print();
53.       }
54.       catch (Exception PrintException) {
55.         PrintException.printStackTrace();
56.       }
57.     }   
58.   }
59.   /**
60.    * Method: print
61.    * <p>
62.    *
63.    * This class is responsible for rendering a page using the provided
64.    * parameters. The result will be a grid where each cell will be half an
65.    * inch by half an inch.
66.    *
67.    * @param g
68.    *            a value of type Graphics
69.    * @param pageFormat
70.    *            a value of type PageFormat
71.    * @param page
72.    *            a value of type int
73.    * @return a value of type int
74.    */
75.   public int print(Graphics pg, PageFormat pageFormat, int page) {
76.     int i;
77.     Line2D.Double line = new Line2D.Double();
78.     Graphics2D g2d;
79.     //--- Validate the page number, we only print the first page
80.     if (page == 0) {  //--- Create a graphic2D object a set the default parameters
81.       g2d = (Graphics2D) pg;
82.       g2d.setColor(Color.black);
83.       //--- Translate the origin to be (0,0)
84.       g2d.translate(pageFormat.getWidth()/2-width/2, pageFormat.getWidth()/2);
85.       pg.drawImage(g.image, 0, 0, null);
86.       return (PAGE_EXISTS);
87.     }
88.     else
89.       return (NO_SUCH_PAGE);
90.   }
91. } // printerclass

Good luck!
Rolf

Thanks, it works almost perfectly for me.