How to make custom terrain?

Im already done the art for it (creep from starcraft) and im trying to replace blight with creep. Now how do I import it so that it works? When I try it in the editor, it comes up as normal blight.

I've also found out that blight is a bugger to replace, i think i did it once but i cant remember what patch it was so try all of these;

TerrainArt\Blight\Blight.blp
or
TerrainArt\Blight\*Blight.blp
(replace * with the start of the blight you are using e.g. LordsBlight for lordaeron)

Textures\Blight.blp
and just try
TerrainArt\Blight.blp

Like i sed, these may or may NOT work, hope it helps.