Sure. Here is a basic example that extracts different kinds of color information (red, green, blue, hue, saturation, brightness). Any and all of those numbers can be used to generate letters. In this example the brightness is converted into a letter. So black is A, white is F and the rest in between. If you want more elaborate note notation, you could replace the chars (letters) by strings (words). If you want a different conversion, change the function brightToTone() in the example below. In the full project you could use the pixel array and for the color of each pixel generate a note.

1. char[] tones = new char[] { 'A','B','C','D','E','F' }; // an array of notes
   
2. 
   
3. void setup() {
4.   size(255,255);
5.   for (int x=0; x<width; x++) {
6.     stroke(x); // from 0 to 255 aka black to white
   
7.     line(x,0,x,height);
8.   }
9. }
10. 
    
11. void draw() {
12. }
13. 
    
14. void mousePressed() {
15.   color c = get(mouseX,mouseY); // gets the color at the mouse location
    
16.   print("\nrgb: " + int(red(c)) + "," + int(green(c)) + "," + int(blue(c))); // extracts and prints rgb
    
17.   print(" | hsb: " + int(hue(c)) + "," + int(saturation(c)) + "," + int(brightness(c))); // extract and prints hsb
    
18.   print(" | tone: " + brightToTone(brightness(c))); // convert brightness to tone and print to console
    
19. }
20. 
    
21. char brightToTone(float bright) { // input a brightness (or any other value!) and get a tone
    
22.   int remap = int(map(bright,0,255,0,tones.length)); // remaps the input value (0-255) to array (0-6)
    
23.   char tone = tones[remap]; // get the tone at this position in the tones array
    
24.   return tone; // return the tone
    
25. }

Yes. Load the image and go through the pixels (pixel array).

Check out: http://processing.org/learning/pixels/