Hi !
Here is a mod that replaces the waving Flags with static ones:
https://www.dropbox.com/s/bq1t4ynnxwr1z ... d.zip?dl=0
Just copy all the files into:
Thirty Years' War\TYW\Graphics\Factions\Cityscape Flags
...and override the existing files (you may want to do a backup of the original files).
Have fun !