You just need to add some styling to that #topAD, something like this...
Something to pay attention to when working with floats - See the box model here. The box model says that whatever you put inside an element cannot be larger than that element. margin/padding/border all count when figuring width/height.
background: transparent url(top_bg.png) repeat-x left top;
margin: 10px 0 0;
background:url(backgroundcontent.png) no-repeat left 105px;