Selectable.targetGraphic

Description

Graphic that will be transitioned upon.

using UnityEngine;
using System.Collections;
using UnityEngine.UI; // Required when Using UI elements.

public class ExampleClass : MonoBehaviour
{
    public Image newImage;
    public Button btnMain;

    void SomeFunction()
    {
        //Displays the sprite transitions on the image when the transition to Highlighted,pressed or disabled is made.
        btnMain.targetGraphic = newImage;
    }
}