Hi

Elk has a good guide here https://forums.triplea-game.org/topic/3435/a-brief-guide-to-making-units-colorize-re-scale-etc

What I have done is use existing units and then tweak their color or add some flames and smoke or remove a special marker/roundel type thing. Pinta or paint.net I think it is for windows and gimp are the two I used.

Sadly they keep getting updated and I have to learn how to use them all over again lol GIMP is the more powerful but also more complex.

I would search existing maps like Iron War and Total World War and see if they have what you're looking for. Frostion and Hepps aren't active anymore but always shared their images when they were, so just make sure to mention them in the credits that it's their units you're using if you do.

Veqryn and Joe Pants both have a pile of units out there and BlackElk has quite a few as well.

I don't have Veqryns units any more but you can do a search for Veqryn Unit Art and see if anything comes up or All Unit Art.

Will ping @ebbe as he makes units and is still active, so he might be willing to help.