The modification you did in the backoffice, changed the pagelayout.tpl from the default sitedesign of your site. In fact the modification is a image class issue. Here is the template code :
The image class called is "logo". But it does not exists for plain_site. You will have to add it.
Image classes are defined in a <i>image.ini.append.php</i> configuration file. you can change this settings in <i>settings/override/image.ini.append.php</i> or <i>settings/siteaccess/plain_site/image.ini.append.php</i> files.
