Creating basically a slideshow but a bit different

[glv]

You did this:

@JoeyS

void draw() {
  background (0);
  image(slides[0], 0, 0);
  
  }

Try this:

int slide;

void draw() {
  background (0);
  image(slides[slide], 0, 0);
  }

And change slide with mouse, keyboard or over time with frameCount() or millis().

The Processing website has resources (tutorials, references, examples) for you to peruse:
processing.org

:)