All the images must be exported as .tga or .tga.gz. All the alpha must be not premultiplied. (It is good idea to avoid semitransparency alltogether)

It is recommended to use ImageMagick command line tool to convert PSD to TGA directly.

    convert terrain.psd -colorspace sRGB -write mpr:temp -background black -alpha Remove mpr:temp -compose Copy_Opacity -composite terrain.tga

'convert' is a ImageMagick's convert.exe, usually located at 'C:\Program Files\ImageMagick-7.0.8-Q16'

If you have ImageMagick installed, please use included batch file.