Sample code for adding Schema.org markup to your website's company logo.

wp-logo-schema.php

<div itemscope itemtype="http://schema.org/Organization">
  <?php 
    $attachment_id = 2; // Upload your client's logo to the Media Library and add the attachment ID here (swap the 2)
    $image_attributes = wp_get_attachment_image_src( $attachment_id );
  ?> 
  <a itemprop="url" href="<?php echo esc_url( home_url( '/' ) ); ?>" rel="home" id="logolink">
    <img itemprop="logo" src="<?php echo $image_attributes[0]; ?>" id="mainlogo" alt="<?php echo get_bloginfo ( 'description' );  ?>" />
  </a>
</div>