That's one of the bugs that Jasc never fixed before moving on to version 9. If this is for WindowBlinds/SkinStudio, your best bet is using png. In SkS. go to Tools > Preferences > Image editors .... check the option "Convert TGA to PNG before using image editors" and any TGA file you edit will be in the PNG format. You can also import PNG files, and they'll be converted to TGA when you Save the skin.
