Which graphics are you referring to? If you mean the menus like this one:
those can be changed directly in a tile editor. You may need to fix the tile map though - that's usually directly after the graphics, and it's prefaced with the width and height values (e.g. 0x0A 0 0x04 0... would say the tile map is 10 tiles long, 4 tiles tall).
If you're referring to the "captions" in the houses in Primary Village (like "Gennai's"):
those are compressed graphics. The compression might match an existing tool, but I ended up building my own that handles imports/exports. I need to rewrite it so it's more user friendly, but if you want, I can send you the original images to edit. You can email them back, and I'll make a patch you can apply to your copy of the game.