Make sure its 256 colors. Heres the skins process:
1. Open TE and save your plane texture.
2. Close TE
3. Open C:Program Files/HTC/aheditor/terrains/yourterrain/savedtx/planebmpyouwant.bmp
4. Make your skin. Save as 256 colored bitmap.
5. Create texsrc folder.
6. Move ALL of your plane skin-related bmps into texsrc.
7. Open TE, open your map, build.
8. Close TE
9. Locate yourmap.res file in the TE main folder and move it into AH terrains.
10. Open your map in AH
I bolded 9 because you didnt mention that.