Without a doubt, the easiest way to do what you are asking is to find the header image that is being used and replace it with your own version of it. In the case of the extremepixels themes you have mentioned, they are very different headers so you need to take care to work within the constraints of their images. The centura is a dynamic set of images so that's not going to be that easy to retrofit your banner to it. Maybe you ought to tell us what you are thinking when you say "How do I add a banner"? Do you just want to lay a banner on top of what's there or replace what there?
