Moving a picture with RGSS3

Hello there!

Having RMVXAce, I'm trying to re-write my Picture Menu script (that worked in RMXP) for this new version.
Basically, when calling the menu, a picture is supposed to appear by moving towards the middle of the screen.
 
However, RGSS3 apparently has a problem with picture displacement: it displays them only at the final co-ordinates, no matter how large is the variable containing the duration of the movement, you can't see their motion at all!


Here's my script for pic motion in RGSS3:

class Game_Picture
 attr_reader :duration         # in order to read the variable of movement duration from anywhere.
end

class Img_Test
  attr_accessor :spriteset
 
 def initialize
   # @wait = 200
    @img = Game_Picture.new(z)
    @img = $game_map.screen.pictures[z]
    @img.show("picture name", origin, x, y, zoom_x, zoom_y, op, blend_type)
    SceneManager.scene.spriteset.update    #to update display
   
 
    @img.move(origin, x, y, zoom_x, zoom_y, op, blend_type, duration)
    SceneManager.scene.spriteset.update
    print @img.duration
    loop do
      @img.update
      SceneManager.scene.spriteset.update
       if @img.duration <= 0
         break
       end
     end
     SceneManager.scene.spriteset.update
   
  end
 
  end

    In RGGS1 (which my script perfectly works with),  $game_map.screen.pictures[z] and SceneManager.scene.spriteset.update must respectively be changed into $game_screen.pictures[z] and
$scene.spriteset.update ......
   
 Has anyone a idea to help me out?