If you look at the comments at the top of Game_Picture, "It's used within the Game_Screen class"
Look at the Game_Screen class, and you'll see on line 34 that 100 "Game_Pictures" get created.
And on line 144, each picture gets updated once per frame (on the map).
Game_Picture is only the object that holds the information about each picture. Not the picture (bitmap) itself.
So, back to Game_Picture...
Look at the 'show' method (line 60..) Notice that @name get set to the first parameter in the def line, "name"
By assigning a name to the Game_Picture object, the Sprite_Picture object (which loads the bitmap) knows to show the picture. (You can also set all the other parameters using this method too.) So, to load "MyPic.png" into the 1st picture, you use...
Game_Screen.pictures[1].show(name, origin, x, y, zoom_x, zoom_y, opacity, blend_type)
of course, replacing all the arguments with values...
Game_Screen.pictures[1].show("MyPic", 0, 120, 60, 100, 100, 255, 0)
Or, to create a picture using all the default values...
Game_Screen.pictures[1].name = "MyPic"
Be Well