I was about to post something along the same lines as to the meaning of the cover.
I don't like the pose, but I can now see that there
is an artistic intent in the cover. It's titled My Story and has a key on the front. As was pointed out, that implies that we are unlocking her diary, her story. So it makes sense to have a simple cover with just a close up shot of Ayu. I just think that the pose could be more flattering. Like the cover of Inspire for instance, that was a great close-up shot of Ayu.