Trying to make randomly generated collages in Processing

I’m really new to Processing 3 and I’m really enjoying it but also about to rip my hair out! I am wanting to to create random collages that are built from .png assets. The code works fine at the moment and does (half) of what I want it to do.

I was wondering if anyone could help me with the following please?

1. Applying a random positioning property to the images so they appear randomly across the canvas so they are not just in a line
2. The same as above but a variable size
3. Adding a code that stops the same image from duplicating in one collage

Thanks so much in advance, here is my code so far:

PImage[] images = new PImage[67];
int[] counters = new int[5];
int DISPLAY_TIME = 4000; 
int lastTime; 
int number = 0;
 
 
 
void setup() 
{ 
  size(600, 600); 
  for (int i = 0; i < images.length; i++) 
  { 
    images[i] = loadImage("Collage_" + nf(i+1, 2) + ".png"); 
    images[i].resize(100,100);
    
 
  } 




  lastTime = millis(); 
 
}   
 
void draw() 
{ 
  background(250); 
  fill(250);   
  if (millis() - lastTime >= DISPLAY_TIME)
  { 
  for( int i = 0; i < counters.length; i++){
  counters[i] = int(random(0,images.length));
}
lastTime = millis();  
  } 
for( int i = 0; i < counters.length; i++){
  image (images[counters[i]], 40 + 30 * i, 50);}
}
 
 
void keyPressed(){
  if(key == 's'){
    println("Saving...");
    String s = "screen" + nf(number,4) +".jpg";
    save(s);
    number++;
    println("Done saving.");
  }
}