By the way why do you prefer TGA instead of TIFF or other losless formats?
I think that it was just easiest for our coders to write a .TGA loader module to our engine years ago, so that decided the format and it has stuck. No reason really why it couldn't be .tif or something else with alpha capability (.tif has alpha channel too, right?)
And i guess you won't talk about your normal map compression?! ;O)
No. :)
...But these should give you some clues:
http://download.nvidia.com/developer/Papers/2004/Bump_Map_Compression/Bump_Map_Compression.pdf
http://www.ati.com/developer/NormalMapCompression.pdf