GUIStyle.active  

// Assigns a texture to customStyles[0] for when the control
// is pressed down

var aTexture : Texture2D;

function OnGUI() {
if(!aTexture) {
Debug.LogError("Assign a texture on the editor first");
return;
}
if(GUI.skin.customStyles.Length > 0)
GUI.skin.customStyles[0].active.background = aTexture;
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
public Texture2D aTexture;
void OnGUI() {
if (!aTexture) {
Debug.LogError("Assign a texture on the editor first");
return;
}
if (GUI.skin.customStyles.Length > 0)
GUI.skin.customStyles[0].active.background = aTexture;

}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

public aTexture as Texture2D

def OnGUI():
if not aTexture:
Debug.LogError('Assign a texture on the editor first')
return
if GUI.skin.customStyles.Length > 0:
GUI.skin.customStyles[0].active.background = aTexture