Unity: Gallery Images Random are repeating

I have a gallery Image where i have 4 UI images choose random sprite form my sprite array and assign it after a random time. My issue is that my 4 UI images are repeating for example Image 1 will display House, there is also a change for Image 2 , 3 or 4 to display the same image. I don't want that. my gallery script is attached to each UI image there for i have 4 UI images so i have 4 instances of the script. issue is that the UI images doesn't Know about each other therefore i tried to get component of Gallery Script component so I can get the value and not make it choose it but sadly it didn't work. Could someone Help me? Here's my Gallery script

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class Gallery : MonoBehaviour
{

public float timer;
public Sprite[] gallerySprites;
public Image currentImage;

public int newRandomNummer;
int lastRandomNumber;
int min = 0;
int max;

public Gallery[] galleries;

public List<int> val = new List<int>();

// Use this for initialization
void Start()
{
    max = gallerySprites.Length;
    timer = MakeRandomNumber(8, 18);
}

// Update is called once per frame
void Update()
{

    timer -= Time.deltaTime;

    if (timer <= 0)
    {


        StartCoroutine(fadeImages());
        timer = MakeRandomNumber(8, 18);


    }

}

IEnumerator fadeImages()
{
    currentImage.CrossFadeAlpha(0, 1f, false);
    yield return new WaitForSeconds(1f);
    currentImage.CrossFadeAlpha(1, 1f, false);
    currentImage.sprite = gallerySprites[MakeRandomNumber(0, max)];
}


int MakeRandomNumber(int min , int max)
{
    newRandomNummer = Random.Range(min, max);
    for (int i = 0; i < galleries.Length; i++)
        val.Add(i);
        if (newRandomNummer != lastRandomNumber)
        {
            Debug.Log("new Value");
            return newRandomNummer;
        }

        else
        {
            //retry!
            newRandomNummer = Random.Range(min, max);
            return newRandomNummer;
        }

}
}