10lire, you said you could need a tuorial about adding flags. It's a while ago that I had to deal with that problem, but I am pretty sure that one or two of these "how to add a new civ" tutorials also cover the flag problem. Sorry, but I have no time to search the right tutorial for you.
Don't know what kind of flags you plan to do... If you want to make a poly colour flag (flag with more then two colours), you need to switch the <bWhiteFlag> tag to 1. Also you may need to play around with the alpha channel. Some users get white flags in the game if the flag has an alpha channel and the flag works normally if it has none. For other users it's exactly vice-versa. AFAIK there is no way to create a flag that works for all users. The easiest way to add an alpha channel is by converting the flag with DDS converter, but that program is buggy. I think there is a description about how to add an alpha channel in one of those tutorials I mentioned above.
Unfortunately I can't help you with bicolour flags, I never made such flags.