Example 1

You        : Excuse me. Do you mind taking us pictures?
Stranger  : Sure?
You        : Thank you. You can just press the button here.

Example 2

You        : Excuse me. Can you take pictures for us please?
Stranger  : Certainly. Is everything set?
You        : Please move to the left a bit. Thank you.

Example 3

You       : Excuse me. Can you do me a favor?
Stranger : Sure.
You       : We'd like to ask you to take a picture for us.
Stranger : ok.... All right, ready, one... two....
You       : Thanks a lot

Example 4

You       : Excuse me. Do you have time to take a picture for us?
Stranger : Oh. I am sorry. I really have to go.
You       : It's ok.