You can usually incorporate taking a pic with your camera phone when you are number closing if you do it right. If a girl is asking you why then you say " well I have to put a pic to the number so when you call me I know who is calling " They will understand that a majority of the time, but I would only try it if you feel you built a lot of comfort.
Or even a quick "Come on".. put arm around her and take a picture of both of you and save it as the pic to go with the number right there in front of her. Most likely she will do the same thing with you with her phone.