Once a computer is upgrade to Mavericks it is a somewhat complicated procedure to install an older OS. You can't just install the OS (like going from Lion back to Leopard).
I happen to think that Snow Leopard is the best OS currently available ... but if she wants the computer primarily for photos then the OS doesn't make that much difference. So given the difficulty of reverting to a pre-Mavericks OS I suggest you just leave it as it is.
If you are determined to revert, then if at all possible make an appointment with a genius at an Apple Store and let them do it for you.