Try this, although I'm not sure if it works (put this anywhere above main).
class Game_Picture
 attr_reader  :target_zoom_x
 attr_reader  :target_zoom_y
 attr_reader  :duration
 def zoom_x(x, duration)
  @duration = duration
  @target_zoom_x = x.to_f
 end
 def zoom_y(y, duration)
  @duration = duration
  @target_zoom_y = y.to_f
 end
end
To use it, call script and have :
$game_screen.pictures[pictureidhere].zoom_x(number here, duration in frames here)
You can either put a number in "number here" and "duration in frames here" or a variable, like $game_variables[yourvariableid]