When I take a portrait picture on my N900 and share it (ie. facebook or evernote) the image is sent as a landscape image. The phone seems to know how it should be rotated but the receiver does not.

anyone else having this problem and perhaps has a solution?

a rotate before sending option might be of use.

It is also true if you open the photos on windows, but on linux they are properly oriented.

windows and linux maintain the rotation differently? anyone know where the difference lies?

This sounds like Bug 9511. Please vote for that bug. Add a comment if you can give decent click-by-click instructions on how to reproduce this bug, but otherwise don't add a comment as bugs.maemo.org isn't a forum.