It's quite simple, just download Wc3viewer here (if u don't have it):
http://www.wc3sear.ch/index.php?p=Tools&ID=11&sid=09d2e480c2ece99afce62ea4e77bdaf9
Search for TerrainArt in the "Search for MPQ" alternative, and you'll get a list of Tilesets to edit.
Now, open the tileset and press the "Extract and convert" button, and then open it with a painting program (even Paint works fine). Then edit it the way you want, go to Wc3viewer and do "Convert BMP/JPG/TGA to BLP", select your edited tile, and convert it.
Now open the TFT World editor, and import your BLP-tile through Import Manager, change path-name to TerrainArt\<Name of your tile>.blp . Make sure that the path is the same as the one in the "Search for MPQ" Window. Now, save the map, restart the WE, and if you did all fine, you should be able to see your Tileset instead of the tile you edited.
I think this would work, i've never tried myself, but it should be like skinning.