a search wouldn't hurt on this one... so many people have asked about this
have a look here for instance:
http://forum.bsplayer.org/viewtopic.php?t=702
to sum up:
try changing one of your codecs/filters settings, one of them most probably has a
flip option. If none of them has, install ffdshow, it will decode most of your movies and has a flip option.
codecs/filters properties are accessible via right-click on the movie window-->properties in bsplayer