We may get the x-ray of the skull, but in all the shots of Frank Castle, not once does he bear the Punisher's insignia on his chest. The skull is very important for a couple of reasons: 1) it looks damn cool, and helps spread fear throughout the underworld, instantly recognisable as the sign of the Punisher, and more importantly 2) it's intended to effectively act like a target, as criminals are drawn towards shooting at it rather than his face, leaving him relatively unarmed due to wearing something like Kevlar. Generally the Punisher is introduced already having the skull, but it appears as though that's not going to be the case here, meaning at some point he's going to have to come across the image and start painting it on his chest, which may be the show's way of representing his full transformation into the Punisher.