New UI, Simple Button Image Swap C#

Unity Engine
1

I can not for the life of me create a simple button image swap when pressed in the new UI.

Basically I have two images. Picture a running man icon, and walking man icon. All I want is the Walking man icon to display when the app loads, then when the Walking man is clicked, it turns into a Running man icon. Then when the Running man icon is displayed and clicked, it turns back to the Walking man icon. Other functions will be added later.
A simple image swap of the “Image (Script) - Source Image” inside my “Button” gameObject.

But nothing I’m creating inside of C# or the Unity Editor is working. This is how I’ve started my code but not sure how to finish it and make it work with the new buttons:

using UnityEngine;
using System.Collections;
using UnityEngine.UI;
using UnityEngine.EventSystems;

public class Buttons : MonoBehaviour {
    public Sprite runningMan;
  
    // Use this for initialization
    void Start () {
        runningMan = Resources.Load("Run.png", typeof(Sprite)) as Sprite;
        GetComponent<SpriteRenderer>().sprite = runningMan;
    }

How do I finish this to work with my Button??

2

id recommend checking out the source code (https://bitbucket.org/Unity-Technologies/ui/overview) and looking at our implementation of button.