This is something I've considered in the past, and will still consider, but I will say that the banner is the key visual for the entire forums, and a very important part for me in how the site is represented. It's the face of the forums and it gives the initial impression to people who visit the site. So it's very important to make sure that I feel it does a good job with that.
We have a team of graphic makers that currently handle making the banners for the forums and all of them get approved by me. This is part of the reason I don't open up submissions, if I were to not chose a person's header to add to the line up, which is currently made for the next 6 months plus some birthdays, it could cause hurt feelings, which is something I would really like to not have to do.
While it's not a no! it's just something that is very personal to me and something may be down the road, but isn't currently in the books.