// 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