I take the picture in portrait orientation on my phone (Google Pixel 3 XL)
I transfer it to my computer via USB file transfer, it stays in portrait orientation on my computer.
I drag and drop it into the post.
It adjusts it to landscape orientation.

I have tried;
changing the rotation on my computer (90 degree and 270 degree)
renaming the file

Steemit changes it to landscape at 90 degree rotation in every case.