What I like:
* Image orientation (from EXIF) support
* I like how intuitive it is
* I will absolutely use some minor ideas in my product that I have not considered using.
What I dislike:
* I have uploaded 10000 x 6672 image (this one https://unsplash.com/photos/2S4FDh3AtGw) and I see that you scale it down. My experience shows that both Firefox and Chrome handles such large image without problems.
* Delay between action and reaction is too high for my taste. It feels unnecessary sluggish. UPDATE: tested with Chromium and it feels response. Only Firefox is sluggish.
* Flip does not center around zoom
Anyway, awesome work as I know some pain points you might have experienced :)