Noggin module allows you to upload a custom header image on the Theme Settings page, just as you can logos and favorite icons. With Bartik and other core Drupal themes, the header image will appear automatically. For themes that use slightly different markup, you can enter in a custom CSS selector for the header region.
This is an excellent module to use as an example for Drupal's core extendability (a module modifying a theme! --even if simply by adding CSS.)
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