This is a contradiction. I am a freelance photographer. I want to take a group of photos for a girl for free, but what should I say?

Actually, it's very simple. This kind of thing just doesn't want her to think much. On the surface, if you take a picture of her, and she has no money to give it to you, she will feel embarrassed and inappropriate, and she will owe you a personal favor later, so there is a high probability that she will refuse you.

but if you change the way, it will be easy to turn this matter into a matter of asking for help. You should organize it yourself, and say that you asked her for a favor, invited her to dinner later, and owed her a favor. Generally, girls are soft-hearted, and there is a high probability that they will promise you.