TGA

TGA is short for Targa (aka Truevision), a non-proprietary image format favoured by Valve. Base format for VTF, and used to store images used by the Steam client.

TGA files can be saved with a fourth channel, called the Alpha channel, which usually defines the transparency. (The three basic channels define red, blue, and green content of the image.) However, VMF shaders can use the alpha channel for other information.