Hi Carrie!
Thank you for visiting the Moms Panel!
Disney's PhotoPass Service is very easy to use. There is no charge for having your photos taken, and you do not need to register before your trip. PhotoPass photographers are located in all the four theme parks, Downtown Disney and at the two water parks during the summer months. When you have a photo taken, the photographer will scan the card (which is the size of a credit card) and your photo will be recorded on that card. Each time you have a photo taken by a PhotoPass photographer you can hand them your card, which they will scan, and hand back to you. When you return home, you can register your unique card number on the PhotoPass website, view your photos and order/pay for any number of these within a 30-day period.
If you forget to take your PhotoPass with you on a particular day, no worries, they will simply give you another one. At the water parks they will give you a waterproof card which can attach to your swimwear for safekeeping. Again, when you arrive home you can register all your cards to your PhotoPass account.
Usually the photographers are happy to take an extra photo with your own camera if you so wish - just ask!
If you have any more questions, please come back and ask me - I would be happy to help you plan you magical vacation!