I've only made about 4 or 5 asks for photos, DC, NYC and CHI dudes, and have never been told no. (Had one guy simply not respond.) I don't ask until we've texted on availability, etc. I also don't hire unless we've had a brief (3-5 min) convo before, and ask them to send it just before the call. (The request for a call does occasionally put a guy off, but I don't think I want a guy who sounds like Harvey Fierstein). Maybe I'm lucky (or over-vet), but I've never had a problem.
I give them the option of three fingers or flipping me off. No one's chosen the latter yet.