Check out pixels[], loadPixels() and saveStrings(). Between these you should be able to split an image into it's seperate pixel-values and save them to a .txt. And after you created the file, the inverse process should be achievable with related commands.

Colors themselves are already ints that can be saved to a text file... if for some reason you want to!

You should also store the dimensions of the image in the text file for reconstruction.

Code Example (saving pixels to text file)

1. void setup() {
2.   PImage img = loadImage("http://rosswarner.com/Dilwyn2med.jpg");
3.   savePixels(img);
4.   exit();
5. }
6.  
7. void savePixels(PImage img) {
8.   String[] pix = new String[img.pixels.length+1];
9.   pix[0] = img.width + "," + img.height;
10.   for (int i=0; i<img.pixels.length; i++) {
11.     pix[i+1] = "" + img.pixels[i];
12.   }
13.   saveStrings("pixels.txt", pix);
14. }

Code Example (loading pixels from text file)

1. PImage img;
2.  
3. void setup() {
4.   img = loadPixels("pixels.txt");
5.   size(img.width, img.height);
6. }
7.  
8. void draw() {
9.   image(img, 0, 0);
10. }
11.  
12. PImage loadPixels(String filename) {
13.   String[] pix = loadStrings(filename);
14.   String[] dimensions = split(pix[0], ',');
15.   int width = int(dimensions[0]);
16.   int height = int(dimensions[1]);
17.   PImage img = createImage(width, height, RGB);
18.   for (int i=1; i<img.pixels.length+1; i++) {
19.     img.pixels[i-1] = int(pix[i]);
20.   }
21.   img.updatePixels();
22.   return img;
23. }

i have written one code... kindly check... the problem is while reconstruction i am getting mirror and rotated image..

reconstruction code:

BufferedReader reader;
size(333,333);
reader = createReader("rea2.txt");
String lines[] = loadStrings("rea2.txt");
println("there are " + lines.length + " lines");

PImage img = createImage(333,333, RGB);
img.loadPixels();
for (int i=0; i <lines.length; i++)
{
String ma = lines[i];
int[] nums =int(split(ma," "));
img.pixels[i] = color(nums[0],nums[1],nums[2]);
}
img.updatePixels();
image(img,0,0);

PImage img;
PrintWriter output;
void setup() {
size(350, 350);

output = createWriter("rea2.txt");  
img = loadImage("getParts_02.jpg");
img.loadPixels();
loadPixels();
  for (int x = 0; x < img.width,x++)
{
  for (int y = 0; y < img.height; y++ ) 
{
int loc = x + y*img.width;
    r =int( red (img.pixels[loc]));
g =int(green (img.pixels[loc]));
b = int(blue (img.pixels[loc]));

String s = r+" "+ g+" " + b+" ";
output.println(s);
}
}
updatePixels();
}
void keyPressed() {
output.flush(); // Writes the remaining data to the file
output.close(); // Finishes the file
exit(); // Stops the program
}