First switch the capslock off ~,~
The Am Tileset Editing methode is the worst way to change the tilesets.
It works, but you have a good chance to crash your rom, because of the file size at the import step.
Use the unLZ GBA methode to change your tilesets, its saver and easier. You may need to run/make a "deep scan" and you have to do all in B/W (it can be with the right palette to but its a bit hard)
but when you write the stuff back to your rom you can give the tileset a new offset (when your going to wright your tileset back to the rom, give the tileset a new empty "image Offset") and thats the problem of the AM methode.
By inserting things you can overwright stuff that make your game messed up.
sry to say that ".