PImage img;

void setup(){
  size(200,200);
  img = createImage(width, height, RGB);
  
  // just filling the image with stuff. replace this with loadImage(filename);
  img.loadPixels();
  for(int i = 0; i < img.pixels.length; i++){
    img.pixels[i] = color( 0, i/255, i/255 );
  }
}

void draw(){
  background(100);
  
  PGraphics maskImage;
  maskImage = createGraphics(img.width, img.height, JAVA2D);
  maskImage.beginDraw();
  maskImage.fill(200);
  maskImage.noStroke();
  maskImage.ellipse(mouseX, mouseY, 50, 50);
  maskImage.endDraw();
  
  PImage imgCopy = img; // if the mask changes every frame, we need to use a copy of it, keeping the original intact
  imgCopy.mask(maskImage);
  image(imgCopy, 0, 0);
}

2. PImage myImage;
3. PImage myMask;
4. 
   
5. color fgColor;
6. color tColor;
7. 
   
8. void setup()
9. {
10.   myMask = loadImage("mask.png");
11.   
12.   tColor = color(0,0,0,0);      //transparency
13.   fgColor = color(255,255,255); //color in your mask that is foreground
14. }
15. 
    
16. void draw()
17. {
18.   myImage = getImageFromSomewhere();
19.   
20.   cropImage();
21.   
22. }
23. 
    
24. void cropImage()
25. {
26.   //assumption here is that both myImage and myMask are the same size!!!
27.   
28.   int w = myImage.width;
29.   int h = myImage.height;
30.   
31.   myImage.loadPixels();
32.   myMask.loadPixels();
33.   
34.   for(int r = 0; r < h; r++)
35.   {
36.     for(int c = 0; c < w; c++)
37.     {
38.       color px  = myMask.pixels[r*w + c];
39.       
40.       if(px != fgColor)
41.       {
42.         //set transparent
43.         myImage.pixels[r*w + c] = tColor; 
44.       }
45.     }
46.   }
47.   
48.   myImage.updatePixels();
49.   myMask.updatePixels();
50. }