JPG vs TGA
This is only from my experience and not based on any text book theories or
formulae.
The problem with using JPG, even at full quality, vs. targa, even RLE
compressed targas is when you have to go through several codecs to reach your
end product. When you store the jpg and then open it up again you go through
one codec process. If you then use that jpg in a work and render it again,
saving to a new jpg file you have two codecs. The affect of this multiple
codec can be severe, much worse than true lossless compression such as RLE. I
have found that the name of the game for lossless is to only use one codec in
the project chain which should be the final phase.
(codec: compression-decompression)