I use a very simple way to edit my GIF file:
I open Units.GIF on Paintbrush (yes, I use the free windows software, and nothing more).
Then, I just copy one of the "empty" squares from the lower part of the file. I choose the BIG zoom, and edit the unit pixel by pixel. The results are awesome.
I created many good units with this method. I'm using none of them now because I got tired of custom units and returned to the standard ones.
Here are some of my former custom units, and you can re-create them in your games:
Kamikaze - It's as strong as a Bomber, but half the price of a Fighter. It explodes in the first attack, just like a missile.
Barbarians - Requires no technology. Ut's a bit more expensive than Warriors, but has 2 attack points.
Urban Militia - It requires Conscription. It's cheaper than Riflemen. Defense 4, attack 3.
Junker - A ship made out of junk (old military material recycled into a new unit). Requires Industrialization. It's just like a cheaper weaker Battleship. Costs only 2 shield lines. Attack 4, defense 2. It's only good against Transports and ancient ships.
Martirs - Imagine how would it be a land missile unit. Well... This unit requires Fundamentalism. Movement: 2. Attack: 12. Defense: 1. It explodes when attacks (just like a missile).
Bomb car - Just like the Martirs. But it's a bit more expensive and moves faster. Movement: 3.
Hovercrafts - It's an expensive unit. The Amphibious element of the Marines with the strenght of an Armor.
Bomb Ship - It's a cheap vessel which appears with Explosives. It's like a missile-ship. It explods when attacking. Movement: 7 Attack 20 Defense 3.
I had the idea of creating this unit when I read about an accident in Canada during World War I, when a cargo ship full of gunpowder and dynamite exploded in the docks. It was the biggest non-nuclear explosion in History. The explosion was so strong, that it created a giant wave, flooding 3 nearby towns. Some pieces of the ship were found 4 kilometers from the coast. Half the population of the town became blind just by looking at the explosion.
Sometimes I also disable nuclear weapons, because they just make the world stop all the wars! Everybody gets so scared of mw when I have nukes that the game becomes boring. Instead of nukes, I prefer to add a new custom unit.