Thanks Deon.
As Deon said, creature graphics and the tileset are separate from each other. You can use other creature graphics with this tileset, or another tileset with these creature graphics.
Creature graphics are in the raws, the tileset is in data/art. Data/init/init.txt lets you specify which tileset to use and whether DF should use creature graphics or not at all.
Don't forget to update your raws in the save folder if you change something.
By the way, I made a few changes to the tileset recently, since it was pretty difficult to distinguish between ground tiles on z0 and z-1.
I won't bother updating the downloads for this (since I'll update the anyway when the new version comes out), but here's the updated tileset anyway:
(Save as "CLA.png" in data/art and overwrite if prompted) The two tiles that changed are the floor tile (someone mentioned there's too much black, so I adjusted it a little without hurting the contrast against the walls) and #250 (next to the sqroot symbol).
Someone also mentioned that in some situations you can mistake upstairs for a '7' (water for example). I won't change this because of the various hardcoded limitations it would mean that the up and downstairs would look different from the up/down-stairs, which I don't want either.
Of course everyone's free to edit the tileset how they see fit anyway.