I always made it a point to ask about photos while chatting/interview. Usually if you talk to them for 10-15 minutes and they get a good guy/non-jerk vibe off you, they are okay with it.
If you are nervous, it might help to ask them after you agree on price and are about to leave.. kind of like an afterthought "oh hey, esta bien peudo tomar fotos?" I think that is close enough translation. Or go with just fotos, ok?
I met a cool guy down there that even went so far as asking about videos, but he knew his stuff. Wouldn't pull a girl unless she did bbbj, photos, videos, etc...
I never paid extra, but I would sometimes get them a drink next time i saw them and thank them for the fun time and especially the foto saying I will think about her a lot. Positive reinforcement, hopefully make it easier for the next guy asking for pictures too
