Don't go for something complicated. I tried to paint a kind of demon with a claw reaching out, but it looked awful. If you don't belive me, look at the GW regiments. They hardly use anything more complicated than a tree or an eye for a banner motive. Start a new layer for the writing.
Most banners feature the name of the regiment.
The script-fu functions of GIMP come in handy here. At last you can finish the banner with some effects. I made lightning for the vampires banners. This is very easy to archive. Just paint multiple white or bright blue zig zag lines on the background.