Would something like this, but on mobile work, or is it too subtle?
I sized it to fit in the established space between a line of text and the top of the image. Are you thinking of having a mobile image expansion indicator always visible, or have it appear on some scroll or tap event?
In any case, I really like that this discussion is happening. I’ve been a bit unhappy with the way images, scrolling, and the metadata bar work together on mobile.