Simicro,
The file you need to change is:
Icon0097.shp
Which can be found here:
WinSPMBT\Game Data\Graphics
The US and USMC flags are icons 13 and 14 within the Icon0097 shape file.
This post here should explain how to do it:
http://forum.shrapnelgames.com/showthread.php?t=47320
Let me know if you get stuck.
The process sounds a bit complicated at first, but once you understand it, it's quite easy and should only take a couple of minutes.
Cross