TPL Files
From WADder Wiki
TPL Files are a texture format used on the GameCube and Wii, and are used in Banners. They are fully supported by many homebrew applications, and leaked Nintendo tools are never necessary (aka, don't use texconv.exe).
End User
End users should never need to touch TPL files directly, if you use WADder or MyMenu. If you don't, you can always use Zetsubou (you have to compile it) via the command line for conversion to and from all types of TPL images.
Technical Info
Great technical details can be found in these two places:
