Create a new room. Go to the backgrounds tab and...
Create object island 1; set the sprite and set the depth to 10000, to make sure it's drawn first, all other objects stay above the islands.
Add the Create event so when the room loads, have it moving down at the same speed as the room, Vertical Speed 2.
Add a Step event to check if the island is out of the bottom of the room, to jump to a spot before the top of the room...
For island2 and island3, just duplicate island1 and change the names and sprites.
First, change the grid size to 80 x 80.
Save your game and test it out.