The T-bone and Porterhouse steaks are cut from the same part of the beef carcass, the short loin. They all include a bone with a strip steak on one side of the bone and a filet on the other. All porterhouse steaks qualify as T-bone steaks, but not every T-bone steak is a Porterhouse. In order to be classified as a Porterhouse steak, the USDA requires that the filet be at least 1 1/2" wide. That normally occurs near the rear-most portion of the short loin. Somewhere along the loin, the filet will narrow to 1 1/4" and all steaks cut from that point forward will be classified as T-bone. From your photo, I suspect your steak might qualify as a Porterhouse.
When cooking such steaks, it does not really matter whether you have a T-bone or Porterhouse. Just make sure you do not overcook it.