How to rotate a Texture2D object?
You rotate the object that uses the texture 2D object, never the texture itself
or you rotate it through altering the rotation of the material
You can rotate a texture in your GUI using GUIUtility.RotateAroundPivot, see http://forum.unity3d.com/viewtopic.php?t=39176&highlight=guiutility+rotatearoundpivot