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

using UnityEngine;
using System.Collections;

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

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public someTexture as Texture2D

    def Example():
        guiTexture.texture = someTexture