if you write ask for photo, that becomes your signature for that card. it is a myth that's been passed on the internet for years. do not leave it blank, what ever is wrote there become the legal signature for the card. so basically some could write check id on the signature line of the credit slip and they have to take it. the proper way for a business to take your credit card is to swipe your card, have you sign the slip, then they are suppose to flip over your card and match the signature from the slip to your card. if they don't match they can keep your card and report it as possibility stolen. it is all in the merchant agreement contract that you have to sign in order to accept credit cards at your business. it is best to sign it as soon as you get them with your own signature.
I've been doing this for 15 years and never had a problem. I also hand clerks my card upside down. I've had my card info stolen before, that's a nightmare.
also if you enter a pin number no ID is needed, as far as reading your card number, it used to be real easy to read the numbers as you hand the card to the cashier, but they are trying to make it harder, some have probably noticed when you go to read off your card number for a over the phone or internet purchase that they are harder to read now.