Searching the forum I found it is possible, but I’m to dense or to new to game making (I suspect the former) and can’t do it right.
How do I add images to the image bank? (Or it has noting to do with it?)
How do I fill the background?
How do I reach the floor? (No matter how much I scroll down GD keeps going)
How do I multiply the tiles for the floor?
Is there another way to add (Moving or static) obstacles, platforms and scenery?
Solved with a third party tool Tiled Map Editor it’s multiplatform, free and opensource.
Now every body can use it.
Good, but remember that GD support tiled objects:
http://www.forum.compilgames.net/download/file.php?id=1528
I know, but how do you edit or create a map in GD?
Haven’t been able to do it myself. 
The tiled object is only used to repeat the same image, it is not intented to be used as a full tiled map editor.
But it is useful to create large background and you can combine several objects to create a map: A sprite object to display buildings, tiled objects to create forest, grass background or mountains…