The battle sprites are stored in compressed format in the rom. In other words, to insert "functioning" battle sprites into the rom, you need to compress the images, while you insert them, to make them look proper in the game.
If you were to insert images into the game without compressing them, you could use a tile layer editor for that. AgiXp is a game-specific program that can insert new sprites into the game and compress them with the compression format that G/S/C use. So basically, use AgiXp to do the job.