Ah, so it's the 2d and 3d that makes the difference. And yes, I have not clue how one creates a 3d one.. Oh well.256x128 is the standard for homebrew (possibly the only size for 2D banners? or maybe someone just picked that years ago)
It would be possible to do more if 3D banners were supported, but currently they aren't. It would probably be a relatively simple code change to support them though, the tricky part is just making them.