Like Wlfgng said!
For every tile ( terr00XX.bmp ) that you place in the folder of the terrain you are making, it will automatically replace the original one of the same name.
Original tiles are located in your C:\Program Files\HTC\aheditor\terrains\textures folder.
Copy one ( like snow - terr0016.bmp ) and paste it into your terrain name folder. Make changes to it with paint and then look at the tile while in the editor.
Pokie