I gave you a video link on your Reddit post that goes into more detail if you need it.
Let's assume your green island is a Sprite with the origin at its center with the name Island.
It's a square so you can grab the area with a formula
Create Object at:
X position: Island.X-(Island.width/2)+random(Island.width)
Y position: Island.Y-(Island.Height/2)+random(Island.Height)
I suggest putting that in a function - then whenever you destroy an object you call an object.
I made a quick example program of it - Sprite = island and sprite2 = object you create.
The algorithm does not take into account your objects width and height, so they will hang off the ledge if you end up randomly hitting very near the edges.