It's difficult. Right now, the only people here who can do graphic hacks are the real three rom hackers. I never got past the "Find the sprite in Geiger's Sprite Viewer" stage, and I don't believe anyone else has tried to become proficient in it. But all you have to do is find the relevant packet of data, decompress it, edit and reinsert. I'm sure there are some special stipulations, probably relating to the way CT handles the tiles.
At any rate, it may be some time before we get to 1999 A.D. (our amazing rate of progress notwithstanding), so we'll have time to plan.