Sorry if this question has already an answer somewhere, but I've just started to play with Unity and now I'm stuck. :(

I've tried a few things ( from [here][1] , [here][2] and [here][3] and others ) but none of them work, I'm sure that I do something wrong but I don't know what.

I have this code:

public class SceneManager : MonoBehaviour {

    public Image myButtonImage; 
    private Button switchBtn;

    // Use this for initialization
    void Start () {

        switchBtn = GameObject.Find("switchBtn").GetComponent<Button>();
        myButtonImage = Resources.Load<Image>("switchCameraImg");

        switchBtn.image = myButtonImage;
    }
}

and this are my objects:

[![enter image description here][4]][4]

where cameraSwitchObj is a GameObject and switchBtn is the button itself.

SceneManager is the script attached to MainCamera

I receive this error :

NullReferenceException: Object reference not set to an instance of an object SceneManager.Start ()

for this line : switchBtn.image = myButtonImage; but according to what i read from different posts this is all i must do. What I am missing?
[1]: https://answers.unity.com/questions/1127944/changing-the-image-of-a-button-with-a-script.html [2]: https://stackoverflow.com/questions/30273435/unity-5-0-how-can-change-unity-ui-button-image [3]: https://blog.gamedev.tv/how-to-add-a-background-image-to-the-ui-canvas-of-your-unity-game/ [4]: https://i.sstatic.net/rdWcr.png