Add logo image resolution in img tag
 {% block body %}
        {% block header %}
                <header id="header">
-                       {% block header_title %}<h1><a href="{{ site_url }}"><img src="{{ asset(site_logo) }}" alt="{{ site_title }}" /></a></h1>{% endblock %}
+                       {% block header_title %}<h1><a href="{{ site_url }}" title="{{ site_title }}"><img src="{{ asset(site_logo) }}" alt="{{ site_title }}" width="171" height="32" /></a></h1>{% endblock %}
                        {% block header_nav %}
                                <nav>
                                        <h2>{% trans %}Navigation{% endtrans %}</h2>