I'd imagine some do, there is some nasty fuckers floating around. Some with very identifiable features. Rather than take a chance they may ask. I don't blame them either, some of the shit ladies have to put up with is abhorrent from what some providers have told me. 98% of punters are normal people, might be some time wasters or eejits but harmless. But that small percentage can be downright nasty. Why should they take the chance? Once they are in a room with them it's too late. So if a well known provider asked for a pic I wouldn't hesitate to send one.