I don't think there is set rules when it comes to the banner. I've seen people submit them and they get used (I think the Nyan-Cat Fender was a submission) and I've seen Dragoneer commission them (in the note leaks).
I would like to see them put some standards in place with the new UI though, something like "must incorporate the logo". Mostly because I think enforcing brands is important. Right now, the banners always look out of place and tossed up in the corner without much thought.