How do you ask a girl to have a picture with you?

First of all why do you want picture of a girl with you, without any occasion or party, sometimes when unknown guys come to a girl in a party and ask for a picture with her, she feels irritated. I am also a girl and I have pictures with so many guys in parties or functions on their request or groups, but in the cases they were in my class, college or friends of friends. You cannot approach an unknown girl for a picture with her.