I know exactly what to do! You'll have to rename the files to stuff like:
Canines: Cheadshade,Cbodyshade,Ctailshade(1/2/3/4/5)
Felines: Fheadshade, Fbodyshade, Ftailshade(1/2/3/4/5)
Then you should be able to replace the texture files
You'll need to slightly modify the shades, I recommend Gimp! C/Fheadmask5 is the texture for the nose usually so I'd start with that one. The other ones effect what shows up on head or body. (Whatever you do, DO NOT replace the ftailShade, that causes the game to crash)