Here are some things that need to be done when setting up a MythTV box. These apply to at least MythBuntu 8.10. Your mileage may vary.
Fix the movie poster download
1. You need to update the location used to store the movie posters. By default it points to a non-existent location. Do this by going to Utilities / Setup | Setup | Media Settings | Video Settings | General Settings | Pg 1/7. In the 'Directory that holds movie posters' field, change the path to /home/--USERNAME--/.mythtv/MythVideo. Replace --USERNAME-- with whatever you setup as your default user name. This is the same as whatever your home directory is in a terminal window.
2. Now open a terminal window and create this directory. First cd .mythtv then mkdir MythVideo (case is significant in Linux so be sure both the path in step 1 and this name match in case).
3. Now you need to update the poster download script. Open FireFox (on your Myth box) and visit http://launchpadlibrarian.net/17738115/imdb.pl.gz. This will download a file named imdb.pl.gz. By default it will be on your desktop. Open a terminal window and cd Desktop. Now we need to unzip the script so gzip -d imdb.pl.gz. This will create a file named imdb.pl in the current directory (and imdb.pl.gz will go away). Now we need to move this file to the correct location but first, let's make a backup copy of the original file, just in case. So, mv /usr/share/mythtv/mythvideo/scripts/imdb.pl ~/imdb.pl.original. Now that we have a backup, move the new copy into place mv imdb.pl /usr/share/mythtv/mythvideo/scripts/.