You can now add a custom header background image. It's very easy to do. Go to settings, Customize Interface, and then click the link at the top of the page to upload a custom header image. The upload page has instructions and tips for usage.

Image size should be 1024 pixels wide. The height can be whatever you choose.

You can choose whether or not you still want to display your company name. The links at the top of the header will remain.