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