Thanks for posting this, (I didn't even know there was such a thing as a 'built in' scrollbar!) EDIT: There isn't!

With regard to your question about the if statement, the online reference example has...

float img2Pos = hs2.getPos()-width/2;

i.e. position relative to width - so if you have four images check if which vertical 'quarter' of the screen the scrollbar is in and use that to show the right image....

pseudocode

if position < width/4 image to display = img1
if position >width/4 and position <width/2 image to display = img2
etc

Maybe? :D

I didn't test if your code works, not_55, since it feels like I should be able to do this without using "width". I put the if statement in void update(), and now I get a NullPointerExeption. This can't be the right way to do it either?

Well, you only have the position (i.e. X) of the scrollbar to tell you where it is so you'd have to use that to determine which image to display. I was making the assumption that your scrollbar was the width of the screen which wouldn't necessarily be the case.

So, scrollbar extends in X from X1 to X2 - you just need to see where the scollbar 'button' XB is in relation to the two ends..

X1 =200                                         XB            X2=600

XB is nearest to 3/4 of the way between X1 and X2 so display third image.

I can code a simple example later if you like?

Oh, please do not_55, that would be so kind of you! I will keep working on it myself. I believe there is a problem with the string where I put my images as well, as I get this message with the NullPointerException: 

You do:

1. String path = "C:/Users/Vibeke/Documents/Processing/mine_tester/bildegalleri4/data";
2. String[] img_list = images(path);
3. void setup() {
4.   size(400, 400);
5.   noStroke();
6.   // Inputs: x, y, width, height, minVal, maxVal
7.   bar1 = new Scrollbar(0, height-20, width, 10, img_index, img_list.length);
8.   img = loadImage(img_list[img_index]);
9. }

Try instead:

1. String[] img_list;
2. void setup() {
3.   size(400, 400);
4.   noStroke();
5.   // Inputs: x, y, width, height, minVal, maxVal
6.   bar1 = new Scrollbar(0, height-20, width, 10, img_index, img_list.length);
7.   path = sketchPath("data");
   
8.   img_list = images(path);
9.   img = loadImage(img_list[img_index]);
10. }

I moved the init in setup() because the sketch path isn't known at the global declaration level.
Asking Processing the sketch path is more flexible and less prone to errors.

PhiLho:

you try to load Thumbs.db as a image
delete this file from your folder (it might be hidden and can re-occur) or
check in your program whether is is an image
e.g.

1. if (img_list[img_index] ends with jpg of bmp) ...

Is this what you're trying to achieve? (Create four images as named to try it out)

1. 
   PImage[] myImages=new PImage[4];
   HScrollbar hs1;
   
   void setup(){
    
     size(640,480);
     myImages[0]=loadImage("one.jpg");
     myImages[1]=loadImage("two.jpg");
     myImages[2]=loadImage("three.jpg");
     myImages[3]=loadImage("four.jpg");
     hs1 = new HScrollbar(0, height-32, width, 16, 16); 
   
   }
   
   
   void draw(){
    
     int index=0;
     background(255);
     index=int(hs1.spos/(width/4));
     image(myImages[index], width/2 - myImages[index].width/2, 0); 
     hs1.display();
     hs1.update();
    
   }
   
2. 
   
3. //scrollbar class from here...
4. 
   class HScrollbar {
     int swidth, sheight;    // width and height of bar
     float xpos, ypos;       // x and y position of bar
     float spos, newspos;    // x position of slider
     float sposMin, sposMax; // max and min values of slider
     int loose;              // how loose/heavy
     boolean over;           // is the mouse over the slider?
     boolean locked;
     float ratio;
   
     HScrollbar (float xp, float yp, int sw, int sh, int l) {
       swidth = sw;
       sheight = sh;
       int widthtoheight = sw - sh;
       ratio = (float)sw / (float)widthtoheight;
       xpos = xp;
       ypos = yp-sheight/2;
       spos = xpos + swidth/2 - sheight/2;
       newspos = spos;
       sposMin = xpos;
       sposMax = xpos + swidth - sheight;
       loose = l;
     }
   
     void update() {
       if(overEvent()) {
         over = true;
       } else {
         over = false;
       }
       if(mousePressed && over) {
         locked = true;
       }
       if(!mousePressed) {
         locked = false;
       }
       if(locked) {
         newspos = constrain(mouseX-sheight/2, sposMin, sposMax);
       }
       if(abs(newspos - spos) > 1) {
         spos = spos + (newspos-spos)/loose;
       }
     }
   
     float constrain(float val, float minv, float maxv) {
       return min(max(val, minv), maxv);
     }
   
     boolean overEvent() {
       if(mouseX > xpos && mouseX < xpos+swidth &&
          mouseY > ypos && mouseY < ypos+sheight) {
         return true;
       } else {
         return false;
       }
     }
   
     void display() {
       noStroke();
       fill(204);
       rect(xpos, ypos, swidth, sheight);
       if(over || locked) {
         fill(0, 0, 0);
       } else {
         fill(102, 102, 102);
       }
       rect(spos, ypos, sheight, sheight);
     }
   
     float getPos() {
       // Convert spos to be values between
       // 0 and the total width of the scrollbar
       return spos * ratio;
     }
   }
   

" Could not find a method to load Thumbs.db"
It confirms that showing a full error message instead of truncating it helps greatly those trying to help...
And, yes, I just forgot the path declaration, minor problem for somebody as smart as you.

not_55:

It wasn't ironic, since you did found the solution by yourself.
Thumbs.db is a system file, so it is hidden by default. You can change your Windows settings to allow displaying it, if you want to. But if you remove it, it can be generated again, when looking in the folder with Explorer in thumbnail mode.
Alternatively, you can just skip it when loading the files.

1.     if (pos1>0) { 
2.       img_index++;
3.       if (img_list[img_index % img_list.length].equals("Thumbs.db")) img_index++;
4.       img_index %= img_list.length;
5.       img = loadImage(img_list[img_index]);
6.     }

(still untested, adjust if there is an error... )

Can this part of the code, line 4, be the reason why Thumb.db is in the array:

PhilLho: