Editroid help needed

I'm working on a Metroid graphics hack and I'm using Editroid to add extra tiles to the structures with the structure editor (I'm going to add proper backgrounds). I don't really understand how the structure editor works.



The structure editor is clearly divided into 3 sections. So here are my question:
How do I add custom 16x16 tiles to the second section (they are visible in the first box (at the end of 3rd line and after the "ER" tiles))

Editroid doen't have any kind of manual so this place probably is the best place I can ask.

My guess would be that the three panels are:

1. The collection of 8x8 tiles made of pixels.

2. A collection of 2x2 tiles that you can place in the world, each made up of 4 tiles from panel 1.

3. The world, made up of tiles from panel 2.

Edit: oh, never mind you already seem to know all this. I don't know how to use the editor, sorry.

IIRC, you have to press Ctrl or something while clicking in the window for 16x16 tiles. It's been a while since I've used Editroid, so my memory is a bit rusty.

For future reference, I suggest that you ask at the Metroid Construction forums. They have some more experienced hackers over there. Plus, snarfblam (creator of Editroid) seems to hang around there more than he does here.

You can hold shift to place the selected tile into the metatile (/block/combo/tsa) editor.

Here's some images: http://imgur.com/a/xJWko

Unfortunately there is not enough empty space in the tileset for tiles that the game treats as "air". I don't think adding backgrounds is possible unless you make some complicated palette/tileset hack. Everything is currently rendered behind the background, but since Ducktales hides Scrooge behind the background when entering a secret passage, I don't think it's impossible to do in Metroid.

The easy thing to do would be to simply render Samus on top of the background instead of behind. If you wanted to be clever, you could toggle Samus's sprite priority during door transitions or even introduce something like a tile type that, when Samus touches it, places Samus behind the background. (You would need a bit of a buffer area for that sort of transition.)

That's probably the best way to do it. I need to disassemble and memory map metroid first. I know that there is already source code for metroid, but I want to do it myself.