Download this Blogger template. Click the image above to view the larger image, which is smaller than the actual size. The large image doesn't fill the whole screen but the actual template does.
If you want to use your own image for the header, the image size has to be 900 x 100 pixels. This template can be completely changed from the Fonts and Colors tab in the Layout panel.
Click here to see all the other Blogger templates, or here if you want to learn how to design your own Blogger templates.