data/_site/error_data.php
Add the following line in themes/%yourtheme%/template.php between <div class="container"> and <div class="navbar-header"> around line 35
<a href="/"><?php gpOutput::GetImage('images/logo.png', array('style'=>'float:left; margin-top: 14px', 'alt'=>'logo', ) ); ?></a>
Then login as admin and rightclick the word 'logo' to assign a picture.