【Unity】GUITextureを真ん中で表示する方法２つ ref: http://qiita.com/shori0917/items/2f302bb8badb5e6d5ffe

qiita.cs

void Start () {
    background = GetComponent<GUITexture>().texture;
}

void OnGUI(){
    GUI.DrawTexture(new Rect(Screen.width / 2 - 128, Screen.height / 2 - 128, background.width,   
    background.width), background);
}