Need help with css code for placing affiliate banner in header

I want to place an affiliate banner to the left of my logo on my blogg
Link to blogg

I put this code into my custom_funcion.php

function header_banner_ad() {
	?>

	<div class="header-banner">
<a href="URLHERE"><img src="IMAGEURLHERE"></a>
	</div>

	<?php
}
add_action('thesis_hook_header', 'header_banner_ad');

The result of this was this

Link to screenshot

I also put in an arrow to show where I want to place the banner.
What do I have to add to the code to make it right?