I downloaded Gimp, so I'll see how well I can get that to work... Thanks for all the advice, by the way!
I decided to put up the units I've completed already; you can see them
here. They're in TGAs in a zip, so if you unzip that to your mods directory, you can also see them in the game. Any impressions of which Versions of the units look better (there are 2 Versions of the eels, jellyfish, and alligators) or relative sizes are welcome...