Logistics of a free headshot/portrait day?

I was thinking that if I were to start a business, part time anyway, that a great way to advertise is to set up a portable studio outside somewhere popular like at the mall or local movie theater plaza area (lots of stores, lots of people) and have a free headshot/portrait day and they sign something and put down their email address then I email them pictures and also include my website etc.

Is this a good idea or is there all kinds of legal matters involved?