The banner lives in skin/frontend/default/default/images. So.. you can....
1) Just overwrite the one that’s there.
2) Create another directory under skin/frontend/default/ called whatever you like and put your file in a subdirectory called images. Then go to the admin console and under system/configuration/design in the themes section put the name of your new directory in the edit box labeled “default”
3) Create another directory under skin/frontend called “whatever” with a subdirectory under that called “something” with a directory called images under that and put your header there. Then change the text next to Package to match “whatever” and the text under theme in the default section to “something”.
All three steps above assume you’re just creating a file with a name that matches the magento name - which I think is header_top_container_bg.jpg
There are similar tricks you can do with css by putting modified css files in skin/frontend/default/whatever/ boxes.css is the one I think you want. But that’s slightly harder. Harder still is what I did which is to modify the page template to load custom.css and then putting in css overrides in custom.css