Alright, so I looked back into the database I made for battle backgrounds and I see that the procedure I used to most easily import them involves using GBA Graphics Editor (which is sort of like unLZ) to import only the image (not the palette) into the ROM at a free space offset, and allowing it to auto-fix the image pointers. Then, I hex-edit the original palette used in the game to suit what I'm inserting, select/copy the edited data and back it up (like in a .txt file) and keep it in a folder with the corresponding image. I close the hex-editor without saving so the original data is left untouched, and then I paste the new data I made in free space (usually right after the image data). After that, I manually fix the pointers to the palette and it should work.
If the image is showing up garbled/blank in some way, then the background you are using is probably not indexed or is not aligned properly with the corresponding .raw file in the graphics editor. For fixing alignment, I use Nameless Tilemap Editor, AKA "NTME". Since you were getting a black-screen, either this is the problem or I think it may be that the pointers to the data may be wrong in some way and the game can't even access it.