Do a search: There are several threads on this subject.
Essentially, the girls are divided into two categories:
Those who do and those who don't (let you take pics).
Those who do
These are mostly HGs, who are in HK on short visits.
Most of the pimps let them decide, but for a minority
it is totally verboten. Some girls don't mind body pics
but draw the line at pics showing the face. No extra
charge in 99.5% of cases, but you could show your
appreciation with a larger tip, if you're so inclined.
Those who don't
Generally, walkups won't let you take pics, though
a few don't mind photos that don't show the face.
Doesn't hurt to ask, but then be sure to respect her
wishes. Sneaked pics are basically a no-no on this
forum.
Charm goes a long way. If you have enough
putonghua to put it across, you could offer to let
her delete any photos she finds objectionable.
Good luck.