Switching a GUIbuttons texture when holding your finger on it?

Here is one solution that works flawlessly on my end. You can use touch code, or mouse code. I prefer unity's mousePosition, since I'm just used to it, and it works for mobile as well. In this example (C#) I check if the Rect for my button contains the mouse or touch position. If it does, I check for touch or click. When the button is clicked or touched, the texture changes. In this example, the two texture choices are public and can be assigned in the inspector. I also set the GUI.skin.button background to match the GUIContent image and text, depending on the touch / click condition.

 using UnityEngine;
 using System.Collections;
 
 public class example : MonoBehaviour
 {
     GUIContent buttonContent;
     Rect buttonRect;
     public Texture2D tex1;
     public Texture2D tex2;
     
     //The mouse code in this example works for touch on android.
     Vector2 mouse;
     
     void Awake()
     {
         buttonContent = new GUIContent();
         buttonRect = new Rect(0, 0, 64, 64);    
     }
     
     void Update()
     {
         mouse = new Vector2(Input.mousePosition.x, Screen.height - Input.mousePosition.y);
         if(buttonRect.Contains(mouse) && Input.GetMouseButton(0))
         {
             buttonContent.text = "Touching";
             buttonContent.image = tex2;
         }
         
         else
         {
             buttonContent.text = "Not Touching";
             buttonContent.image = tex1;
         }
     }
     
     void OnGUI()
     {
         GUI.skin.button.normal.background = (Texture2D)buttonContent.image;
         GUI.skin.button.hover.background  = (Texture2D)buttonContent.image;
         GUI.skin.button.active.background = (Texture2D)buttonContent.image;
         if(GUI.Button(new Rect(0, 0, 64, 64), buttonContent))
         {
             //Code here what you want to affect by button press.
         }
     }
 }