If you go to Tools on RMS you should find something called; grftool-win32. If you don't, try to find a download on google.
Then you'll need a sprite bible, you can use the RMS one in conjunction with this:
http://www.geocities.co.jp/SiliconValley-Bay/6314/cnp_sprite_bible_abbreviated.txtSince they both have info the other doesn't.
Then open the GRF tool, use it to look in, I can't remember if it's adata, or sdata, probably s since I guess it stands for 'Sprite'. Anyway good luck finding what you want to edit in there, extract it and use actOR (Should be on RMS too) to extract the individual sprite/item frames to edit in paint/photoshop.
Then use actOR to re-compile it, and well, good luck getting it into the game. I haven't had any luck since my computer is set to Japanese so it doesn't read the data folder names properly. But it shouldn't be..... very hard, right?