I used the old and gold unlz-gba, and the original background is located near +-330. It's something like 40x16, and you can't do much with that. If you import something bigger, it will overlap other tiles when the ROM load them. The thing is that the tilemap will have to be huge, because it will include the professor, some stuff AND the actual background below all of them. The game will load this tilemap above the others, and it will do the job. It's just hard to imagine this without images, lol. I used 32 colors: the actual background palette and the nº1 (hex editing)