Finalement, il vaut mieux carrément cacher l'image de fond de bannière en RWD

stylessoyezcreateurs.css.html

@@ -1176,7 +1176,7 @@ div [lang], span [lang] { font-style: italic; }
  #menu {
    width: auto !important;
  }
-#header { background-size: 100%; height: auto; min-height: inherit; }
+#header { background-image: none; }
 #mobile-header { background-color: #333333; display: block; height: 32px; width: 32px; z-index: 1000; }
 #responsive-menu-button { background: url("http://dev.pyrat.net/plugins/soyezcreateurs/images/nav-icon.svg") no-repeat scroll center center transparent; display: block; height: 32px; width: 32px; }
 #responsive-menu-button span { position:absolute !important; overflow:hidden !important; top:-2000px !important; left:-2000px !important; height:0 !important; width:0 !important; }