Engraved in the forehead are the Citipati, also known as “Lords of the Graveyard”. They are depicted in the “bow and arrow”-posture (elbows and knees intertwined) which referes to the highest grade of the Outer Tantra. The monk who teaches in the “Institut für Tibet- und Buddhismuskunde”, Punchok Namgal, told me that there are seven tantric levels. 3 levels of the Outer, and 4 levels of the Inner Tantra. On the highest grade of the Inner Tantra, the skeletons would be depicted copulating.