1. Go to .../Program Files/Mount&Blade Warband/Modules/Native/Textures or .../Program Files/Steam/steamapps/common/Mount&blade warband/Modules/Native/Textures
2. Back up your files if you want, they are the following:
banners_a.dds
banners_b.dds
banners_c.dds
banners_d.dds
banners_e.dds
banners_f.dds
banners_g.dds
3. Unzip and replace.
4. Enjoy.

Golden Kingdom
The Midnight Watchmen
Nobody Knows
Defenders of Faith
Paladins of the Round Table
Balion Mercenaries
Balion Guard
Knights of Pomerania
Suicide Brigade
Kingdom of Avalon
Christian Peasant Brotherhood
TBD
Bloodlust Soldiers
LES
Ra
West Knights
Malion Mercenaries
The Rebels
Prince Wales Own Militia