this little functions.php hack makes me all giddy…
use this to rebrand your WordPress back-end with your own logo:
//hook the administrative header output
add_action('admin_head', 'my_custom_logo');
function my_custom_logo() {
echo '
<style type="text/css">
#header-logo { background-image: url('.get_bloginfo('template_directory').'/images/custom-logo.gif) !important; }
</style>
';
}
and here’s what it would look like in action:

by default, the WP logo is 32px square. so what do you do if your logo isn’t square? easy enough, just add a width: with an !important tag in the CSS and make it as wide as you like.
via WPBeginner via Smashing Magazine