In my opinion and based on what I have done, the purpose of them asking about the back, is to make sure it is recent, although it is not stated, it is universal that a recent picture should be taken during the last 6 months, now when you get your picture taken by a professional, they will stamp the back using their stamp which includes name of their company, address, date taken and your name. Again this is my opinion cause I don't see the reason they will ask for the back if not to make sure that the picture represents you and that it is recent.