Let me just reiterate what a good theme Graphene is - well done!
I've noticed this issue and I'm wondering if there is a workaround?
If you change the theme width, from the default 960px width to lets say 1200px, the banner settings do not change. What this means is that when you try to upload your now 1200px wide banner, the upload/crop functions force you to crop it to the old 960px width. There is no option to skip the crop.
This is the second time I have done this and the last time eventually the theme got the hint and allowed me to set up a banner of the right width but this time it does not seem to be wanting to work for me. Is there something I'm forgetting?