WordPress/wp-includes/js/tinymce/plugins/wordpress/css/content.css

.mceWPnextpage, .mceWPmore {
	border: 0px;
	border-top: 1px dotted #cccccc;
	display: block;
	width: 100%;
	height: 12px;
	margin-top: 15px;
}
.mceWPmore {
	background: #ffffff url(../img/more_bug.gif) no-repeat right top;
}
.mceWPnextpage {
    background: #ffffff url(../img/page_bug.gif) no-repeat right top;
}

img.wpGallery {
	border: 1px dashed #888;
	background: #f2f8ff url("../../wpgallery/img/gallery.png") no-repeat scroll center center;
	width: 99%;
	height: 250px;
}

img.wp-oembed {
	border: 1px dashed #888;
	background: #f7f5f2 url("../img/embedded.png") no-repeat scroll center center;
	width: 300px;
	height: 250px;
}