// Assign someTexture to the guiTexture
var someTexture : Texture2D;
guiTexture.texture = someTexture;

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
public Texture2D someTexture;
void Awake() {
guiTexture.texture = someTexture;
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

public someTexture as Texture2D

def Awake():
guiTexture.texture = someTexture