Sorry for the late response, I've been quite busy lately. The format of the hangar banners is simply that it should look good as a horizontal banner that will be rescaled to a width of exactly 300 pixels. Many of these should be easy to convert into a square of other format for the new FlightGear GUI. James, if you suggest a format, then we can have graphics created for both purposes and stored on the wiki. A large pixel format in the correct dimensions would be best as it is easier to scale down than up, and it can be used in the future when the GUI changes. Though a true graphics artist will do a lot more than just scaling for different graphics sizes, so some people may like to know the final size.
As for getting a hangar banner up on the wiki, simple set up a wiki account, upload the file (setting the appropriate licence during, or after, the upload), and then update the wiki page [wiki] Template:hangar banner. I have been doing this mainly by myself, but anyone can do this.
Regards,
Edward