I think there’s more than one motivation to ask for the face pic. I’ve met trans men and women and also physically smaller providers who’ve told me that they do that for safety. Although, I’m not sure if they’d know if it’s a fake photo.
I think the bigger percentage of providers who ask for a photo are indeed screening to see if the client isn’t too much of a turn off for them. I’m guessing they are not providers as a full time source of income. I’ve met college students and people with full-time day jobs who take on some extra gig jobs but only when they sound appealing enough.