How to Personalize 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 can upload your own header image and/or change the header text color.
To use existing header images, follow these steps:
Click the Header link in the Appearance menu on your Dashboard.
The Custom Header page appears.
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 predefined dimensions set by the theme you are currently using. For best results, start with an image that meets those dimensions, at a minimum.
Your image uploads to your web server. The Crop Header Image page appears, where you can crop the image and adjust which portion of the header image you would like displayed.
Click the Crop and Publish button.
The Custom Header 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.