I still don't know what rom you're even editing...
Nevertheless, you should probably use some game-specific editor which lets you edit insert new sprites and replace the original ones with those. It's really not worth it trying to edit the sprites "by hand". After all, they're most likely compressed which means that the only way to edit them to make them look good is to actually make an image and insert it to the rom file with a tool that compresses the sprite while at it (so that it's displayed properly in the game where it uncompresses the sprite before it loads it into the screen).
Nevertheless, you should probably use some game-specific editor which lets you edit insert new sprites and replace the original ones with those. It's really not worth it trying to edit the sprites "by hand". After all, they're most likely compressed which means that the only way to edit them to make them look good is to actually make an image and insert it to the rom file with a tool that compresses the sprite while at it (so that it's displayed properly in the game where it uncompresses the sprite before it loads it into the screen).