Using the Admin Parse Output plugin for WordPress

functions.php

/*

 functions.php -  Plugin that this example uses: https://gist.github.com/717514

*/

add_action('admin_parse_output', 'yoursite_admin_parse_output');
function yoursite_admin_parse_output($html) {
	global $pagenow;
	if ($pagenow=='themes.php' && isset($_GET['page']) && $_GET['page']=='custom-header') {
		$msg = '<p>' . __( 'If you don&lsquo;t want to upload your own image, you can use one of these cool headers.' ) . '</p>';
		$html = str_replace($msg,'',$html);
		$html = str_replace(__( 'Default Images' ),__( 'Available Images' ),$html);
		$html = preg_replace('#<tr valign="top">\s*<th scope="row">'.__('Upload Image').'</th>(.*)?</td>\s*</tr>#Us','',$html);
	}
	return $html;
}