BackgroundChanger.cs

using UnityEngine;

public class BackgroundChanger : MonoBehaviour
{
    public GameObject bg1;
    public GameObject bg2;
    public GameObject bg3;
    public GameObject bg4;
    public GameObject bg5;
    void OnGUI()
    {
        if (GUI.Button(new Rect(60, 75, 20, 20), ":)"))
        {
            if (bg1.activeSelf && bg2.activeSelf == false && bg3.activeSelf == false && bg4.activeSelf == false && bg5.activeSelf == false)
            {
                //Debug.Log("first bg is on");
                bg1.gameObject.SetActive(false);
                bg2.gameObject.SetActive(true);
            }

            else if (bg1.activeSelf == false && bg2.activeSelf && bg3.activeSelf == false && bg4.activeSelf == false && bg5.activeSelf == false)
            {
                //Debug.Log("second bg is on");
                bg2.gameObject.SetActive(false);
                bg3.gameObject.SetActive(true);
            }

            else if (bg1.activeSelf == false && bg2.activeSelf == false && bg3.activeSelf && bg4.activeSelf == false && bg5.activeSelf == false)
            {
                //Debug.Log("third bg is on");
                bg3.gameObject.SetActive(false);
                bg4.gameObject.SetActive(true);
            }

            else if (bg1.activeSelf == false && bg2.activeSelf == false && bg3.activeSelf == false && bg4.activeSelf && bg5.activeSelf == false)
            {
                //Debug.Log("fourth bg is on");
                bg4.gameObject.SetActive(false);
                bg5.gameObject.SetActive(true);
            }

            else if (bg1.activeSelf == false && bg2.activeSelf == false && bg3.activeSelf == false && bg4.activeSelf == false && bg5.activeSelf)
            {
                //Debug.Log("fourth bg is on");
                bg5.gameObject.SetActive(false);
                bg1.gameObject.SetActive(true);
            }
        }
    }
}

This script creates a GUI button on the screen which allows the player to change the background image. Using a series of if-statements, I was able to have the button go through all the images chronologically, before looping back to the first.