Events and Pictures do not reside on the same Viewport. In Spriteset_Map:
Try making Sprite_Pictures instantiate on @viewport1.
# Make viewports
@viewport1 = Viewport.new(0, 0, 640, 480)
@viewport2 = Viewport.new(0, 0, 640, 480)
@viewport3 = Viewport.new(0, 0, 640, 480)
@viewport2.z = 200
@viewport3.z = 5000
...
# Make character sprites
@character_sprites = []
for i in $game_map.events.keys.sort
sprite = Sprite_Character.new(@viewport1, $game_map.events[i])
@character_sprites.push(sprite)
end
@character_sprites.push(Sprite_Character.new(@viewport1, $game_player))
# Make weather
@weather = RPG::Weather.new(@viewport1)
# Make picture sprites
@picture_sprites = []
for i in 1..50
@picture_sprites.push(Sprite_Picture.new(@viewport2,
$game_screen.pictures[i]))
end