Joelg - I actually created this one for the national gtg... I do everything in photoshop but you can do it however is easiest for you. The dimensions are in another thread somewhere but once you get the picture banner you want at the allowed size just upload it to a free photo host. I use photo bucket. From there, just copy the URL that it gives you for the picture and paste it into the signature section of your profile editing page.
Or for now, just copy the URL code badtuffstuff gave in his post above and paste it into the signature section of your profile page and advertise for nationals!