I generally ask for a selfie with them holding up a piece of paper with todays date and some word i give them. In my experiene, all the providers who are genuine don't have a huge issue doing this. That way you're not asking for a 'picture' as such either.
All in all though, you can't have the expectation that they'll include their face - and it's perfectly justified for them to want to keep it hidden.
Regardless, just be polite, explain your reasoning behind questions you ask and i they refuse to provide even a faceless pic, then think long an hard about proceeding
