Logo spanning horizontal

You should be able to do this very easily using a child theme… Here I have created one

Go to Appearance > Header and add the image there

@jan very true… I have removed the link and will provide a walkthrough shortly instead

Ok I have put this on github

The three files are
functions.php
header.php
style.css

So Created a normal child theme as per style.css

created a blank functions.php and added

<?php
add_theme_support( 'custom-header' );

Then copied over the header.php from base wp and added to line 43 – 45

<div class="header-image">
      		<img src="<?php header_image(); ?>" height="<?php echo get_custom_header()->height; ?>" width="<?php echo get_custom_header()->width; ?>" alt="" />
      </div>

Hope that helps

You’re welcome