Nine times out of ten, this works for movies I own. Sometimes, it takes a couple tries with different titles.
If all else fails, search for the movie in IMDB, copy the exact title and paste it into the dialog box that comes up when you click 'Enter Movie Title' at the end of the list of suggested movie titles.
On a couple occasions, and in my experience only movies that are rather rare, lesser known, or were made for TV and without a DVD release are without a poster for the movie.
In the above you'll notice "The Game" that's the Michael Douglas movie, I would say; 75% of the title in my movie folder are accurate with regards to the movie posters; and the rest is just rubbish... Do the "read more" and you'll see some weird results.