PImage icon;    // storage for image   

void setup() {    
  background(255); 
  size(400, 400);  
  icon = loadImage("tree.png"); 
  for (int i = 1; i < 4; i++) {
      float scale = .5*i;
      drawTree(scale, i*icon.width, 200);  
  }
}

// Draw the tree image scaled by an amount s 
// and placed so that its center is at x,y
void drawTree(float s, int x, int y) {
  pushMatrix();
  translate(x,y);
  scale(s,s);
  translate(-icon.width/2,-icon.height/2);
  image(icon, 0, 0);
  popMatrix();
}