In your stylesheet you need to replace the following elements with something similar to the code shown below:
#header{float:left;width:100%;margin:10px 0}
#header h1{float:left;margin:0;padding:0;width:950px}
Now you will need to add the following to your stylesheet if the elements are not there already:
#header h1 a{padding:0;margin:0;width:950px;height:150px;display:block;background:url(images/header.png) no-repeat top left}
#header h1 a span{display:none}
The width and height settings above need to be changed to the exact dimensions of your header image.
Now in header.php you will have to find the code that places the name of the blog in the header. This code needs to be replaced with something like this:
<h1><a href=""<?php">/" title="Click for home"><span><?php bloginfo('name'); ?></span></a></h1>
This will give you a header graphic that is clickable at any place on the graphic. The name of the blog will also no longer be there. Hope this helps.