Some people think they can add external resources as they please in your site.

no-ultimate-maintenance-mode-external-resources.php

<?php
/* !---------------------------------------------------------------------------- */
/* ! ULTIMATE MAINTENANCE MODE                                                   */
/* ----------------------------------------------------------------------------- */

/**
 * Make it local or die:
 * Remove the image ad from the settings page (the link stays in place).
 */

add_action( 'settings_page_seedprod_maintenance_mode', 'sf_ob_start', 9 );
if ( ! function_exists( 'sf_ob_start' ) ):
	/**
	 * Start `ob_`.
	 *
	 * @author Grégory Viguier
	 */
	function sf_ob_start() {
		ob_start();
	}
endif;


add_action( 'settings_page_seedprod_maintenance_mode', 'sf_remove_seedprod_external_ad', 11 );
/**
 * Close `ob_` and remove the image.
 *
 * @author Grégory Viguier
 */
function sf_remove_seedprod_external_ad() {
	$contents = ob_get_contents();
	ob_clean();

	echo str_replace( 'http://static.seedprod.com/ads/coming-soon-pro-sidebar.png', 'data:image/gif;base6'. 4 . ',R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7', $contents );
}