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();