Okay, then have you tried taking the textures from here to edit?
http://wowprogramming.com/utils/artbrowser
They're already .png files, and then you just have to save them as .tga in your editing software.
/edit: remember that you need an alpha channel
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"You'd be surprised how many people violate this simple principle every day of their lives and try to fit square pegs into round holes, ignoring the clear reality that Things Are As They Are." -Benjamin Hoff,
The Tao of Pooh