Default Logo for WordPress Custom Logo (since 4.5)
add_filter('get_custom_logo', 'prefix_custom_logo_default', 10);
function prefix_custom_logo_default($html){
if(empty($html)){
$html = '<a href="'.esc_url(site_url()).'" class="custom-logo-link" rel="home" itemprop="url">';
$html .= '<img src="'.get_template_directory_uri() .'/images/logo.png" alt="Logo" class="custom-logo" itemprop="logo" />';
$html .= '</a>';
}
return $html;
}