Simplest Script (OnMouseDown) Wont Work!!!

Hi guys,
I'm having a hard time getting the simplest code to work.
I'm trying to detect the click on a 2d GUI element (image). I have tried absolutely everything I saw on videos and tutorials, but it just does not work.

As a step-by-step, I did the following:
1- New Scene
2- Add Canvas
3- Add Panel to that Canvas
4- Add Image To that Panel
5- New C# script (refer to code below)
6- Added the script to the image.
7- Play & click the image.

When I click the image, nothing happens!! Why??? Why God, Why????? (Sorry for the drama!)

I have also tried adding a collider to that image, but it has had no effect.

Thanks in advance for any insights.

Code (CSharp):

1. using UnityEngine;
2. using System.Collections;
3.  
4. public class TestClick : MonoBehaviour {
5.  
6.     private void OnMouseDown()
7.     {
8.         Debug.Log ("BAZINGA");
9.     }
10. }