I had fixed the desktop problem, but still had the problem with iPad > i think is_mobile is also calling the iPad?
<?php if (is_mobile()) {
$attr = array(
'title' => 'meer info over '.get_the_title(),
'width' => 390,
'height' => 'auto'
); ?>
<a href="<?php the_permalink(); ?>" title="Meer info over <?php the_title();?>"><?php the_post_thumbnail('medium', $attr); ?> <div class="clearer"></div></a>
<?php } ?>
<?php if (is_tablet()) {
$attr = array(
'title' => 'meer info over '.get_the_title(),
'width' => 187,
'height' => 256
); ?>
<a href="<?php the_permalink(); ?>" title="Meer info over <?php the_title();?>"><?php the_post_thumbnail('small', $attr); ?> <div class="clearer"></div></a>
<?php } if ( !is_mobile() ) {
$attr = array(
'title' => 'meer info over '.get_the_title(),
'width' => 187,
'height' => 256
); ?>
<a href="<?php the_permalink(); ?>" title="Meer info over <?php the_title();?>"><?php the_post_thumbnail('medium', $attr); ?> <div class="clearer"></div></a>
<?php } ?>