Is the order in which they were clicked on important?
Can a picture be allowed to be selected more than once?
Do you have some code you've attempted already?

A class template to help you out:

class Picture {
  PImage pic;
  int x, y, w, h;
  boolean isSelected;

  Picture(PImage img, int xx, int yy) {
    spawn(img, xx, yy);
  }

  void spawn(PImage img, int xx, int yy) {
    pic = img;

    setXY(xx, yy);

    w = img.width;
    h = img.height;
  }

  void setXY(int xx, int yy) {
    x = xx;
    y = yy;
  }

  void display() {
    image(pic, x, y);
  }

  boolean isHovering() {
    return mouseX > x & mouseX < x+w & mouseY > y & mouseY < y+h;
  }
}

hello,

easy way without a class:

before setup() declare your variables

1. int [] clickedimages = new int [5];
2. int myIndex=0;
3. int currentImageNumber = 0;
   

to add an image number:
It's necessary that "currentImageNumber" has already the value of the image (0..9) you clicked. Or you enter it by keyboard (instead of mouse), see below.

1. clickedimages[myIndex] = currentImageNumber;
2. myIndex++;

to display all images in clickedimages

1. for (int i.............. ) {
2.     image ( clickedimages[i], i*100, 30);
3. }

Enter image number via keyboard

1. //
2. int max = 5;
3. int [] clickedimages = new int[max];
4. int myIndex=0;
5. int currentImageNumber = 0;
6. //
7. void setup() {
8. }
9. //
10. void draw() {
11. }
12. //
13. 
    
14. void keyPressed() {
15.   if (key>='0' && key <= '9') {
16.     currentImageNumber=key-48;
17. 
    
18.     if (myIndex<max) {
19.       clickedimages[myIndex] = currentImageNumber;
20.       myIndex++;
21.       println (currentImageNumber);
22.       println (clickedimages);
23.     } // if
24.     else {
25.       println ("list full");
26.     } // else
27. 
    
28.   } // if
29. } // func
30. //

Greetings, Chrisir    

If you need an answer, please send me a personal message since this forum doesn't notify.