GUI SpaceShip Movement Problem!!! Please help!!

 using UnityEngine;
 using System.Collections;
 
 public class simpleMovement : MonoBehaviour 
 
 {
     public float speed = 0.00f;
     public float rotSpeed = 10.0f;
     public float strafeSpeed =10.0f;
     private float engineSpeed = 0.0f;
     public float pitchConstant = 1.0f;
     public float yawConstant = 0.0f;
     public GameObject boostersprefab;
     public GameObject boosterPos;
         
     private GameObject instantiated;
     private bool instanBoostersExist;        
     
     // GUI texture
     
     public GUITexture forwardPitchButton;
     public GUITexture backwardPitchButton;
     public GUITexture leftPitchButton;
     public GUITexture rightPitchButton;        
     
     
     
     
     // Booster Start Function
     void boostersStart()        
     {  
         if(instanBoostersExist == false)    
         
         {            
           instantiated = (GameObject)Instantiate(boostersprefab, boosterPos.transform.position, boosterPos.transform.rotation);
           instantiated.transform.parent    = boosterPos.transform;
           instanBoostersExist = true;            
         }        
     }        
     
     // Booster Stop Function
     void boosterStop()
     {        
         if (instanBoostersExist == true)
         {        
             Destroy(instantiated,0.2f);
             instanBoostersExist = false;
         }    
     }
     
         
     
     void Update()
         
     {        
     // Axis Declaration    
     float pitch = Input.GetAxis("Pitch")*pitchConstant;         
     float yaw   = Input.GetAxis  ("Yaw")*yawConstant;        
     float roll =  Input.GetAxis ("Roll");
     float gas =   Input.GetAxis ("Gas");        
     
         
     Vector3 strafe = new Vector3 (Input.GetAxis("Horizontal")*strafeSpeed*Time.deltaTime,Input.GetAxis("Vertical")*strafeSpeed*Time.deltaTime,0);        
             
         // Engine Mechanism        
         
          if (engineSpeed <10 && engineSpeed > -3)
          {
           engineSpeed += gas;            
           boostersStart();          
          }
         
          if(engineSpeed > 10)
          {
           engineSpeed = (9.99f);    
          }
         
          if (engineSpeed<-3)
          {
           engineSpeed = (-2.99f);        
          }
         
           else if(Input.GetKey("z"))// Brake
          {
           engineSpeed = (0.00f);
           boosterStop();          
          }    
     
      
         
         
         
         
     // SpaceShip Physics Movement
         
     rigidbody.AddRelativeTorque(pitch*rotSpeed*Time.deltaTime,yaw*rotSpeed*Time.deltaTime,roll*rotSpeed*Time.deltaTime);
     
     rigidbody.AddRelativeForce(0,0,engineSpeed*speed*Time.deltaTime);    
         
     rigidbody.AddRelativeForce(strafe);    
         
         
     }    
 }

How do I use the GUI button press (already I have tried some ways, but am not able to achieve the effect) which is been assigned with respective texture to move the ship in yaw and pitch position. I have found the way to achieve this using keyboard controls, Please some one help me to fix this code for GUI texture. Thanks a lot, If you can please explain a bit, It would be of immense help. Thanks