Lots of discussion around this issue here : PoC correction horizon avec IMU Witmotion - #15 par qhess34
It’s in French but your browser’s translation helper should allow to understand 
I also asked about similar things a few weeks ago, and the consensus seems to be that’s it’s better not to reproject images before uploading, as there is inevitably a loss of quality when doing this (+ it requires lots of computation on your side before upload). The 360 viewer should be able to read image metadata and adjust the orientation. Right now, this works quite well in Panellum but not so well in Panoramax. I have no idea if this is something that is being worked on. Just like a lot of things in IT, almost everything is possible with time, money and the right people.