Why cant I change the image to another?

Hi All

I tried to setup a small class in my game that allows me to change the target reticle when pressing mouse left and right buttons; but I still cannot get the image to change. I also tried to use a co-routine to create a short pause between changing images, but this doesn't work either - I had to use a co-routine as the update() does not return a value.

here is my code below:

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

using System;

using UnityStandardAssets.CrossPlatformInput; using UnityStandardAssets.Utility; using Random = UnityEngine.Random;

public class reticalChanger : MonoBehaviour {

 public Sprite[] images;
 public Sprite sprite;

 private Vector2 movement;
 


 // Use this for initialization
 void Start (){

     this.transform.GetComponent<UnityEngine.UI.Image>().sprite = sprite;

     
     //IEnumerator Pause()
     //{
     //    print(Time.time);
     //    yield return new WaitForSeconds(5);
     //    print(Time.time);
     //}

 }

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


     if (Input.GetButtonDown("Fire1"))
     {
         sprite = images[1];
         Debug.Log("fire button");
     }
     if (Input.GetButtonDown("Fire2"))
     {
         sprite = images[0];
         Debug.Log("fire button 2");
     }
     /*UnityStandardAssets.Characters.FirstPerson.FirstPersonController FPC = GetComponent<UnityStandardAssets.Characters.FirstPerson.FirstPersonController>();
     if (FPC.Vector() == Vector3.zero)
     {
         //must use IEnumerator as Update does not return a value
         StartCoroutine(Pause());

     }*/

 }

}