【Unity】GUITextureの画像を真ん中に表示する２つ方法 ref: http://qiita.com/shori0917/items/f32f9fcbce165c18e623

center.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);
}