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take us through the stages that get you from taking a picture to adding it to a post, that might yield a clue.

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.

hmm... weird have you tried clicking "selecting them" and choosing it from a folder rather than dragging & dropping?

or using a different browser?

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