Yeesh.
Err.
I assume you want to import some custom cliff textures?
If you import it to... ReplaceableTextures\Cliff\Cliff0.blp
then it should replace the natural red rock cliff. The problem IS, it would also replace several other cliffs...
B = Barrens
C = Felwood
F = Lordaeron Fall
G = Underground
I = Icecrown
K = Black Citadel
N = Northrend
O = Outland
Q = Lordaeron ?
W = Lordaeron Winter
X = Dalaran
Y = Cityscape