This is the image format file that Maya and VTEX.exe can deal with , only TGA are accepted by the converter (VTEX) to export into the VTF format file.
VTF is the format of texture files that can be read by the SourceEngine.
Important : The TGA files must to be stored and loaded from the %GAME_PROJECT_SRC%/materialsrc/ directory.
Note : you can use sub directory, vtex. exe will create the same folder hierachy into the %GAME_PROJECT% directory.