Try this:

CSS...
<style type="text/css">
div.topLeftImg {
background:url('http://domain.com/imagefilenamehere.jpg') no-repeat;
display:block;
width: /*whateversizeitis*/;
height: /*whateversizeitis*/;
position:absolute;
top:0;
left:0;
z-index:999;
}
</style>
And at the top of your HTML/index/template page:
<div class="topLeftImg"></div>