Changing Images at Different Instances. C# script

Hey All, I am trying to make a questionnaire type of game where my questions keep changing after a GUI button is pressed, Since I am not that great with coding(just started learning), So I am having a little trouble with arrays. I want to load a different image for a different question so I did create a script for changing questions using booleans

     public Texture2D[] img;
 
     float timeleft = 15.0f;
     public static int nonmed ;
     public static int med ;
     public static int commerce ;
     public static int arts ;
     bool startq = true;
     bool a = false;
     bool b = false;
     bool c = false;
     bool d = false;
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
         timeleft -= Time.deltaTime;
         if (a) {
             startq = false;
         }
         if (b) {
             a = false;
                     }
         if (c) {
             b = false;
         }
         if (d) {
             c = false;
             Application.LoadLevel(2);
         }
         if (timeleft <= 0) {
             Application.LoadLevel (2);
         } 
     }
     void OnGUI(){
         GUI.Label (new Rect (10, 10, 100, 100), timeleft.ToString ());
                 if (startq) {
         
                         GUI.Label (new Rect (50, 50, 900,800), "");
                         if (GUI.Button (new Rect (550, 400, 50, 30), "A")) {
                                 nonmed += 1;
                 timeleft = 15.0f;
                                 a = true;
                         }
                         if (GUI.Button (new Rect (700, 400, 50, 30), "B")) {
                                 med += 1;
                 timeleft = 15.0f;
                                 a = true;
                         }
                         if (GUI.Button (new Rect (550, 500, 50, 30), "C")) {
                                 arts += 1;
                 timeleft = 15.0f;
                                 a = true;
                         }
                         if (GUI.Button (new Rect (700, 500, 50, 30), "D")) {
                                 commerce += 1;
                 timeleft = 15.0f;
                                 a = true;
                         }
     
     
                 }
 }