Hi Keri,
This is such a great idea! I have no doubt you will end up with some gorgeous pictures.
My suggestion for a great photograph with Cinderella Castle in the background is
Disney's Polynesian Village Resort. The beaches provide a lovely atmosphere with Magic Kingdom park just across Seven Seas Lagoon. If you go earlier in the day you should have the sun behind you as well.
I suggest doing an image search online with the words, "view of Magic Kingdom park from Polynesian" to get an idea of what your photo might look like.
If you have some time at the Walt Disney World Resort and want a few more options, feel free to hop off the Resort Monorail and walk around Disney's Contemporary Resort and Disney's Grand Floridian Resort & Spa for other great views. You do not need reservations at these Resort hotels to tour them.
Wishing you magical memories on your vacation, Keri! Let us know if you have any other questions.
Andrea U.