What does this really mean?
On Windows, uncompressed RGB32/RGB24 images are normally stored upside down (or bottom-up) in memory, unlike YUV images which are stored "normally" (top to bottom). However, applications/codecs have a way to request/propose uncompressed RGB images in both orientations, and they indicate this by the sign of the height. For RGB32/24 positive height means bottom-up orientation, negative height means normal (top-to-bottom) orientation.
In short, it's a technical detail, as both the app and codec know the orientation by looking at the sign of the height, and they will handle/display the image correctly, so the end user doesn't see any difference.
More here: https://msdn.microsoft.com/en-us/library/windows/desktop/dd407212(v=vs.85).aspx
