Here is a photo of the actual prop poster that was hung up in Seth Cohen's room (above his bed). Yakuza Prep was of course a fictitious anime movie created just for the show. This exact poster design can also be seen in Josh Schwartz's other baby and hit TV show "Chuck".
Fun fact: Yakuza is a Japanese word for: a Japanese organized crime syndicate similar to the Mafia.
a Japanese gangster or racketeer.