Can I / Shall I / Should I / Let me / Lets / Why don't I / Why not let me / Is it ok if I /.. Take a photo / picture of you (two) together. Actually in this situation English is extremely flexible. If you say as you suggest " Could you take a picture with B''.. you are asking requesting that person A takes a picture with B and B is unknown. Unless you are suggesting A takes a selfie with B.

"Could I take a picture of you and B ?" [your second version] OR "Could I take your picture with B?"

Shall I take a picture of you and B together?

I would suggest: Could you stand next to B so that I can take a picture of the two of you?

>I would like to take a picture with my friends A and B >(only A and B in the picture,and I am a photographer) must be to take a picture _of_ my friends