can not set instance of ui image in script : using UnityEngine;

using UnityEngine.UI;

using System.Collections;

using Image = UnityEngine.UI.Image;

using UnityEngine.UI;

public class changeimage : MonoBehaviour {

    public UnityEngine.UI.Image imageobject;

   // public Sprite myFirstImage;

    Sprite myFruit = Resources.Load("watermalon", typeof(Sprite)) as Sprite;

	// Use this for initialization

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

	void Update () {
	
	}

    public void ClickToChange2()

    {
        //imageobject = GetComponent<Image>();
        
        //Debug.Log(imageobject);


            imageobject.sprite = myFruit;

            
        
        

    }
}

**Error: Null reference to imageobject**