Hello, Kaitlyn,
You are correct. Only an original, valid passport may be used for identification and age verification when a non-United States resident purchases alcohol at the Disneyland Resort. This is according to the rules of bona fide identification set up by the California Department of Alcoholic Beverage Control. Cast Members do not have the flexibility to vary from these rules. Unfortunately, a photo of a passport would not satisfy the requirements for identification for non-United States residents, even if accompanied by other forms of identification.
I understand your concerns about carrying a document as valuable as your passport. When I'm traveling internationally, I try to be careful like that too. Fortunately, the Disneyland Resort is among the safest travel destinations I've ever experienced or can imagine. That said, it's still wise to use best practices in personal safety and security regardless of where you are in the world. If things like a securely-designed bag or backpack or large, secure pocket don't happen to be options for you, another way you might safely secure your personal belongings, including your passport, would be to rent a locker at the Disneyland Resort. Once rented, you may continue to return to your locker throughout the day. It can be a convenient place to store not only valuable documents, but also extra layers of clothes, purchases made while visiting the Disneyland Resort, and other personal belongings.
I hope that helps as you make plans to visit the Disneyland Resort, Kaitlyn. Please feel free to visit planDisney again if you have any more questions. We're happy to provide planning support and recommendations for dining, the 70th anniversary celebration, attractions, shopping, and more.
Have a magical time, Kaitlyn!
~Tony