GUI関数ではなくGUILayout関数を使う
http://answers.unity3d.com/questions/8727/how-do-i-center-things-on-screen-using-guilayout.html
http://docs-jp.unity3d.com/Documentation/Components/gui-Layout.html
Texture continueWindowImage; Rect windowRect = new Rect (0,0,UnityEngine.Screen.width, UnityEngine.Screen.height); GUILayout.BeginArea (windowRect); GUILayout.FlexibleSpace(); GUILayout.BeginHorizontal(); GUILayout.FlexibleSpace(); GUILayout.Box(continueWindowImage,GUILayout.MaxWidth(UnityEngine.Screen.width), GUILayout.MaxHeight(UnityEngine.Screen.height)); GUILayout.FlexibleSpace(); GUILayout.EndHorizontal(); GUILayout.FlexibleSpace(); GUILayout.EndArea();