FlashCam.cs

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

public class FlashCam : MonoBehaviour {

    public GameObject flashImage;

    // Use this for initialization
    void Start () {
        flashImage = GameObject.Find("Flash Image");
        flashImage.SetActive(false);
    }

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

    }

    IEnumerator flash()
    {
        for (int i = 0; i < 24; i++)
        {
            if(flashImage.activeSelf)
            {
                flashImage.SetActive(false);
            }else
            {
                flashImage.SetActive(true);
            }
            yield return null;
        }
    }

    void OnGUI() {

        if (GUI.Button(new Rect(10, 70, 50, 30), "Flash"))
           StartCoroutine("flash");

    }
}