Documentation : V3.2

This is just a standard TGA. I'll document it here anyway though.

Header
byte	ID Length
byte	Colormap
byte	Image Type
word	First Colormap Entry
word	Colormap Length
byte	Bits per Color Entry
word	X Origin of Image
word	Y Origin of Image
word	Image Width
word	Image Height
byte	BPP
byte	Image Descriptor

ID Length is the number of bytes in the ImageID block.
If Colormap==1, then the colormap block is present
Image types: 0 = No Image, 1 = Uncompressed Indexed, 2 = Uncompressed RGB, 3 = Uncompressed B&W, 4 = RLE Indexed, 5 = RLE RGB, 6 = RLE B&W.
Image Descriptor: bits 0-3 = # of attrs per pixel, bit 4 = Left/Right flipped, bit 5 = top/bottom flipped, bits 6-7 = 0

Immediately following the hearder is the ImageID block, which describes authoring information among other things. This is then followed by the colormap, if present.
Now the image data starts, it's format depends on the settings in the header. Following that is the DeveloperData which includes program-specific data, then the extension area. All of which can be ignored.