Dear Gianpaolo,
Please following the steps:
1. go to your_site\templates\sj_plus\layouts\main-left-right.xml and find:
<positions>
<position class="span4" data-tablet="span4" type="feature">@logo</position>
<position class="span3" data-tablet="span4" data-mobile="span4" style="none" type="modules">social</position>
<position class="span3" data-tablet="hidden" data-mobile="hidden" style="none" type="modules">search</position>
<position class="span2" data-tablet="span4" data-mobile="hidden" style="none" type="modules">contact</position>
</positions>
=>
<positions>
<position class="span2" data-tablet="span4" type="feature">@logo</position>
<position class="span10" data-tablet="span8" data-mobile="span8" style="none" type="modules">social</position>
</positions>
2. go to Administration >> Module Manager >> click button New >> Select a Module Type:Custom HTML >> insert your banner in this module >> chose position : social
Thanks!