the TGA is optional, and for display in the mission briefing. It's how they make a map or image of the engine telegraph appear among the text.
It enhances the briefing, but it's not strictly required.
http://www.subsim.com/radioroom/down...o=file&id=4420
is a mission I made for SH4. It appears as one of the Submarine School missions, but it's got a dead simple briefing with a TGA image. You could pick it apart for more info.
ADVANCED: While my example above simple uses a whole image with no alpha channel on the screen, it's possible to use an alpha channel to create transparent parts (like you could make a round image that way) - or you can put multiple things in one image and tell the game to only show one part in one place and another part in another place.
To research this, take a look at how images are used in the menus.