In WordPress, for example in the TwentyTen theme, you may want to add a new header image to the default options available. This is useful for example if you are copying this theme, to use as the basis for a new theme.

To do so, first upload the image you want to use, and a thumbnail of it to the wp-content/themes/NEW_THEME_NAME/images/headers directory.

Then, in wp-content/themes/NEW_THEME_NAME/functions.php edit the register_default_headers function, to replace ‘sunset’ below with your new image details:

'sunset' => array(
'url' => '%s/images/headers/sunset.jpg',
'thumbnail_url' => '%s/images/headers/sunset-thumbnail.jpg',
/* translators: header image description */
'description' => __( 'Sunset', 'copied_twenty_theme' )
),


Then go to Appearance -> Theme -> Headers in your Admin panel, and you should be able to select the new image as header.