Just want to add another method to this post, and others if looking:
1.
In Appearance>Header
Select “Remove Header”
2.
In Appearance>Editor
Select template Header (header.php)
3.
Add something like this:
<div class="site-add-header"><img class="header-img" src="wp-content/uploads/2013/10/Logo_header1.png" alt="muesli 101 logo"></div>
Click “Update File”
4.
Now select to Edit Stylesheet (style.css)
Do a CTRL-F (Find) “site-header”
5.
Remove
.site-header {
background-image: .....;
}
Add
.site-add-header {
text-align:center;
}
.header-img {
width:80%;
}
You now have a scalable image based site-header for the theme Twenty Thirteen.