On my Windows mobile, I am used to hit back button to close all things that look like modals, like those enlarged images.
I make an exception with Discourse where I have learnt after many times that I would go with back to the previous page instead of closing that image.
So yes, maybe it is not completely consistent with the rest, if I expected it to close image. 
But I am really impressed with how slick is the general Discourse experience, it is really incredible.
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