To get .tga files to show up, they have to be in a certain format. There's an excellent guide over on the WowAce forums about this:
http://forums.wowace.com/showthread.php?t=15439
Note that you might need to be registered and logged into the forums there to see the pictures.
edit: If you have a Curse login, you can use it on WowAce for the forums, btw.