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EDIT: It didn't stop working for no reason, I actually edited img3 = loadImage("3.jpg");  and forgot about it, "3.jpg" doesn't exist, lol.
I managed to make the images move across the screen, but then I wanted the images side by side instead of on top of each other, so I changed image(this.nr, this.start, 0); to image(this.nr, this.start+width, 0); to image(this.nr, this.start+(width*this.nr), 0); 
this.nr is the first var in these lines: slider = new Slider(0,img0);
So I was changing it to and fro, trying to make it work and all of a sudden it said "1426(or something): (something about not being able to load the image or something)", So I saved the sketch, restartet the program and now there's no error, but when I run the sketch it says "loading..." in the middle of the screen.
Here is my sketch.js:
var img1,img2, img0, img3;
var slider;
function preload(){
    img0 = loadImage("0.jpg");
    img1 = loadImage("1.jpg");
    img2 = loadImage("2.jpg");
    img3 = loadImage("3.jpg");
}
function setup(){
    createCanvas(800,600);
    slider = new Slider(0,img0);
    slider1 = new Slider(1,img1);
    slider2 = new Slider(2,img2);
    slider3 = new Slider(3,img3);
    //mover = new Mover();
}
function draw(){
    background(60);
    slider.update();
    slider1.update();
    slider2.update();
    slider3.update();
    slider.display();
    slider1.display();
    slider2.display();
    slider3.display();
    //mover.display();
    print(slider.c);
}
And here is the function I was working on in slider.js:
this.display = function() {
        stroke(255);
        fill(this.p);
        rect(this.start + (width * this.nr), 0, width, height);
        this.test += 1
        image(this.nr, this.start, 0);
    };
Answers
solved?
Yes