The API seems to want the movie in the q
param so change this
variables.name = "Pulp fiction";
to :
variables.q = "Pulp fiction";
To verify : http://imdbapi.org/?q=Pulp%20Fiction
From there getting the poster URL is just a matter of reading the correct property from the JSON string.
private function onComplete (event:Event):void {
var data:Array = JSON.parse(event.target.data);
if(data && data.length)
{
var movie:Object = data[0];
trace(movie.poster);
}
}