How to Customize the Header Image in Your WordPress Blog
Several themes in the WordPress.com library support the Custom Header feature, which allows you to change the image displayed in the header (or top) of your site through the use of a friendly interface on your WordPress Dashboard.
Click the Header link under the Appearance menu to load the Custom Header page where you have eight premade images to choose from, along with the capability to upload your own header image.
With the Twenty Eleven theme, WordPress did all the hard work for you. Including a custom header image on a blog that uses the Twenty Eleven theme is pretty darn easy, whether you use your own header image or one of eight existing images that WordPress provides for you.
To use existing header images, follow these steps:
Click the Header link in the Appearance menu on your Dashboard.
The Your Header Image page appears.
Choose from one of eight header images.
Scroll down to the bottom of the page and select one of the cool header images that WordPress provides by clicking the circle to the left of your chosen image.
Click the Save Changes button.
The header image you chose is now saved. When you view your website, you see it displayed at the top.
To upload a new header image, follow these steps:
Click the Browse button under the Upload New Header Image title. Select the image from your computer; then click Open.
WordPress automatically resizes your header image to 1000 pixels wide and 288 pixels high — so for best results, start with an image that meets those dimensions, at a minimum.
Click the Crop and Publish button.
The Header Image page appears, and your new header image is now displayed.
View your website.
Your new header image is now displayed at the top of your website.
In themes that don’t have the custom header image feature, you can easily define a background image for the header image using CSS.