Hi Donna,
I am super excited for you visiting Disney World for the first time! Its the perfect place to celebrate every occasion. Be sure to pick up a "1st Visit" button a Guest Relations.
Disney's PhotoPass is a great way to get some great shots taken by professional photographers. Its also nice for everyone in your party to be a part of the photo.
When you return from your vacation, you can go online to view your photos. Before you place your final order, you'll be given the opportunity to add borders, character signatures, captions and other unique things to your pictures. If you add a character, it will be added somewhere along the outside of the photo but it doesn't give the appearance that you posed with that character. You will have a chance to preview the amended photo before you save it. You can do as many things with each photo as you'd like - its lots of fun to play with this feature.
There are some places where a PhotoPass Photographer may get you to hold your hands a certain way or face a certain way. In these instances, you will see that a character was superimposed into the photo (i.e., cradling baby Simba in your arms, holding a bouquet or balloons or holding Tinker Bell in your hands).
You won't be able to change the pictures themselves but you will be able to add some really neat things to each of them. Have a great time experimenting with your photos!
Wishing you many magical memories :-) Allison