Well there's no need to be worried about it - if you're comfortable sending a face picture (and some ladies do, even if they don't show their face in their ad pictures) then send one, and if not just politely tell them that you don't have any face pictures as you don't show them. You'll get a few whining, but they're not worth worrying about (and far more people would say no than yes to this).

Either way, don't let anybody badger you into anything you're not completely happy with - don't forget that once that picture has left you it could wind up anywhere (for this reason if you do decide to send them, make it a fully dressed picture in a neutral setting that could have come from anything) .