I was viewing an image in Photos. I'm not sure how, possibly by swiping upwards a certain way, I caused a small thumbnail of the image to appear in the upper left corner.
I have not been able to successfully replicate this action and I am trying to determine whether it is a bug or whether it is a hidden viewing feature.
Before (not sure why but in my mobile browser image appears rotated 90 degrees counter clockwise in this post):

After (screen grab):
