//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>
   ';
}

First, create an image that fits the allocated space. As of WordPress 2.8, the logo is a 30 pixels wide and 31 pixels tall transparent GIF. When using a transparent GIF or 8-bit PNG, ensure that the image matte matches the header background color: hex value 464646.

A logo named “custom_logo.gif” inside the template directory’s image subfolder can substitute the default WordPress logo with the following code inside the theme’s “functions.php” file.

Thanks to Smashing Magazine.

How to replace the default WordPress “W” logo in the administrative header with a custom alternative

wordpress

PHP