open your map in the terrain editor.
Click File, make map
Then open the bmp created called map in a paint program (I suggest something other than MS Paint)
Create a new folder in your terrain folder called texsrc
Save the bmp you opened in the texsrc folder, make sure it is saved as 256 color (8 bmp).
also make sure when you save it in the texsrc folder it has the same name as your .oba file.
Any other textures/skins will also be placed in the texsrc folder, but they have to be saved as 256 color or they will not show up.