1. final int Image_NB = 3;
2. PImage img1;
3. PImage img2;
4. PImage img3;
5. int img =  int [Image_NB];
6. void setup()
7. {
8. size(500,350);
9. smooth();
10.  img1 = loadImage("1.jpg");
11.  img2 = loadImage("2.jpg");
12.  img3 = loadImage("3.jpg");
13. }
14. void draw(){
15.   background(255);
16. }
17. void mousePressed(){
18. if(mouseX > 20 && mouseX<40 && mouseY > height/2 && mouseY< height/2+50){
19.   for (int i = 0; i < Image_NB; i++)
20.   {
21.    
22.   image(img[i], 0, height/6,640,360);
23.   fill(150);
24.   rect(20,height/2,20,50);
25.   rect(width-40,height/2,20,50);
26.   }
27. }
28. else if(mouseX > width-40 && mouseX<width-20 && mouseY > height/2 && mouseY< height/2+50){
29.   for (int i = 0; i < Image_NB; i--)
30.   {
31.    
32.   image(img[i], 0, height/6,640,360);
33.   fill(150);
34.   rect(20,height/2,20,50);
35.   rect(width-40,height/2,20,50);
36.   }
37. } 
38. }

1. final int Image_NB = 3;
   PImage[] img = new PImage[3];
   int i;
   void setup()
   {
     size(500, 350);
     smooth();
     img[0] = loadImage("1.jpg");
     img[1] = loadImage("2.jpg");
     img[2] = loadImage("3.jpg");
   }
   void draw() {
     background(255);
     image(img[i], 0, height/6, 640, 360);
     fill(150);
     rect(20, height/2, 20, 50);
     rect(width-40, height/2, 20, 50);
   }
   void mousePressed() {
     if (mouseX > 20 && mouseX<40 && mouseY > height/2 && mouseY< height/2+50) {
       i++;
       if (i>=Image_NB-1) {
         i = Image_NB-1;
       }
     }
     else if (mouseX > width-40 && mouseX<width-20 && mouseY > height/2 && mouseY< height/2+50) {
       i--; 
       if (i<0) {
         i = 0;
       }
     }
   }
   

maybe not the most elegant way. Ask if some points are still dark.

1. void mousePressed() {
2.   if (mouseX > 20 && mouseX<40 && mouseY > height/2 && mouseY< height/2+50) {
3.     i++;
4.     if (i>=Image_NB-1) {
5.       i = 0;
6.     }
7.   }
8.   else if (mouseX > width-40 && mouseX<width-20 && mouseY > height/2 && mouseY< height/2+50) {
9.     i--;
10.     if (i<0) {
11.       i = 2;
12.     }
13.   }
14. }

1. if (mouseX > 20 && mouseX<40 && mouseY > height/2 && mouseY< height/2+50) {
           i++;
           if (i>Image_NB-1) {
             i = 0;
           }
         }
   

1. final int Image_NB = 5;
2. PImage[] img = new PImage[5];
3. int i;
4. void setup()
5. {
6.   size(600, 360);
7.   smooth();
8.   img[0] = loadImage("1.jpg");
9.   img[1] = loadImage("2.jpg");
10.   img[2] = loadImage("3.jpg");
11.   img[3] = loadImage("4.jpg");
12.   img[4] = loadImage("5.jpg");
13. }
14. void draw() {
15.   background(255);
16.   imageMode(CENTER);
17.   image(img[i], width/2, height/2, 600, 360);
18.   fill(150);
19.   rect(20, height/2, 20, 50);
20.   rect(width-40, height/2, 20, 50);
21. fill(255);
22. beginShape();
23. vertex(37,height/2+1);
24. vertex(21,height/2+25);
25. vertex(37,height/2+49);
26. vertex(39,height/2+49);
27. vertex(23,height/2+25);
28. vertex(39,height/2+1);
29. endShape(CLOSE);
30. fill(255);
31. beginShape();
32. vertex(width-39,height/2+1);
33. vertex(width-23,height/2+25);
34. vertex(width-39,height/2+49);
35. vertex(width-37,height/2+49);
36. vertex(width-21,height/2+25);
37. vertex(width-37,height/2+1);
38. endShape(CLOSE);
39. fill(255);
40. noStroke();
41. }
42. void mousePressed() {
43.  if (mouseX > 20 && mouseX<40 && mouseY > height/2 && mouseY< height/2+50) {
44.         i++;
45.         if (i>Image_NB-1) {
46.           i = 0;
47.         }
48.       }
49.   else if (mouseX > width-40 && mouseX<width-20 && mouseY > height/2 && mouseY< height/2+50) {
50.     i--;
51.     if (i<0) {
52.       i = 4;
53.     }
54.   }
55. }