@ronald122789 got a solution for You. Just ran on a same problem during some experimenting. Came accross to this:
http://talk.maemo.org/showthread.php?t=74980&page=71

So, the solution for You is not to set portrait_wallpaper to true, but to false instead. So:
gconftool-2 --set /apps/osso/hildon-desktop/portrait_wallpaper -t bool false
should do the trick for You.
Will try to get wallpapers rotating and I'll come back with some news
Greets and Good luck!