Well, the bitmap filename is $data_system.title_name, but if you are talking in the actual Scene_Title,
@sprite.bitmap = RPG::Cache.<foldername>(<filename>)
Foldername is just the folders in your graphics folder - s.
Spoiler
RPG::Cache.animation(filename, hue)
Gets an animation graphic. Use hue to adjust its hue values.
RPG::Cache.autotile(filename)
Gets an autotile graphic.
RPG::Cache.battleback(filename)
Gets a battle background graphic.
RPG::Cache.battler(filename, hue)
Gets a battler graphic. Use hue to adjust its hue values.
RPG::Cache.character(filename, hue)
Gets a character graphic. Use hue to adjust its hue values.
RPG::Cache.fog(filename, hue)
Gets a fog graphic. Use hue to adjust its hue values.
RPG::Cache.gameover(filename)
Gets a "Game Over" graphic.
RPG::Cache.icon(filename)
Gets an icon graphic.
RPG::Cache.panorama(filename, hue)
Gets a panorama graphic. Use hue to adjust its hue values.
RPG::Cache.picture(filename)
Gets a picture graphic.
RPG::Cache.tileset(filename)
Gets a tileset graphic.
RPG::Cache.title(filename)
Gets a title graphic.
RPG::Cache.windowskin(filename)
Then filename is just the image name.