Reddit mentions of The Twilight Saga: The Official Illustrated Guide

I wish she would make an encyclopedia. I hate Pottermore. I want something like what Cassandra Clare did or (sorry!) Stephenie Meyer did. I think if she got a design team behind her it would be amazing. These books are amazing and the designers would kick ass at helping her. 1 2 3

First: you are trying to resolve an issue that most computer users won't experience.

Second: this issue wouldn't even exist if you weren't using pictures.

If you stored those treasured moments in word form, instead of picture form, you wouldn't have to hide them. These files could even be stored on the desktop.
She would see the text and either confuse it for a part of the masterworks of Stephanie Meyers, or she would think "Ew. Reading is yucky.".

If she ever asked, you could just simply say "Honey, those are the magical pieces of paper that make your sugarbear's computer box gears turn."

This explanation is even more likely to be accepted if you repeat it to her in a way that makes it sound like you are speaking to a baby. Doing this correctly will cause her current psychological frame of reference to "regress" to an earlier state, in which she will simply trust everything you say, implicitly.