According to this site, it said the rotation is because
u took the photo with phone under "rotation lock" mode, especially when the problem persists after you rotated the photo before posting.
Prease test again with your ATB gf later without the "rotation lock" mode
"This happens because the picture was taken with a mobile device (phone or tablet) in landscape orientation (horizontally) with the “rotation lock” mode enabled. When this is the case, an order to automatically rotate the picture is stored on the photo’s metadata. Turns out, some parts of the dashboard or the mobile app cannot read image metadata. "
https://forum.hootsuite.com/discussion/6615485/photos-always-posting-sideways