This script will automatically scan an entire directory and retrieve meta data including posters for tv and movies.
http://www.mythtv.org/wiki/Fill_mythvideo_metadata.pl
A recent update is here:
http://fill-mythvideo-metadata.googlecode.com/svn/trunk/fill_mythvideo_metadata.pl
There is a line to set an external script to download movie posters.
#Set as 0 if you want to take posters from IMDB,
#or 1 if you want to generate from file.
#or 2 if you want to take posters from an external script
my $movies_snapshot = 0;
my $external_poster_command_line = '/usr/share/mythtv/mythvideo/scripts/tmdb.pl -P '
Also see the discussion thread
http://ubuntuforums.org/showthread.php?t=1061733&page=23