What's the avatar image of the user who created the movie list "250 Favourite Films"?
250 Favourite Films refers to list_title; avatar image refers to user_avatar_image_url;
SELECT T2.user_avatar_image_url FROM lists AS T1 INNER JOIN lists_users AS T2 ON T1.list_id = T2.list_id AND T1.user_id = T2.user_id WHERE T1.list_title = '250 Favourite Films'