Thank you Uwe
To amend try replacing the contents of your templates header.php (../templates/[YourTemplate]/php/layouts/header.php) with the following...
<?php 
defined( '_JEXEC' ) or die( 'Restricted index access' );
 ?>
<div id="logo">
  <?php if($this->params->get('logoImage') == '1') : ?>  
  <div class="logo logo-image"> <a href="/<?php echo $this->baseurl ?>" title="<?php echo $siteName; ?>">
    <?php if($this->params->get('logoimagefile') == '') : ?>
      <img class="logo-image primary-logo-image" src="/<?php echo $this->baseurl ?>/templates/<?php echo $this->template?>/images/logo.png" alt="Logo" />
    <?php elseif($this->params->get('logoimagefile') != '') : ?>
      <img class="logo-image primary-logo-image" src="/<?php echo $this->baseurl ?>/<?php echo $logoimagefile; ?>" alt="Logo" />
    <?php endif; ?>
    <img class="logo-image tablet-logo-image" src="http://maass-kfz.de/images/logo/logo_t.gif" alt="Tablet Logo" />
      <img class="logo-image phone-logo-image" src="http://maass-kfz.de/images/logo/logo_m.gif" alt="Mobile Logo" />
    </a> </div>
  <?php else : ?>
  <div class="logo logo-text">
    <h1> <a href="/<?php echo $this->baseurl ?>" title="<?php echo $this->params->get('siteName'); ?>">
    <?php echo $this->params->get('logoText'); ?>
    </a> </h1>
    <p class="site-slogan"><?php echo $this->params->get('sloganText'); ?></p>
  </div>
  <?php endif; ?> 
</div>
<?php 
$document->addStyleDeclaration('
.tablet-logo-image, .phone-logo-image {display:none;}
@media only screen and (max-width: 1024px) {
  .primary-logo-image {
    display:none;
  } 
  .tablet-logo-image {
    display:inline-block;
  }
}
@media only screen and (max-width: 767px) {
  .primary-logo-image, .tablet-logo-image {
    display:none;
  } 
  .mobile-logo-image {
    display:inline-block;
  }
}
');
?> 
Ciarán