EDIT: I found the solution, but now there is a problem with cakePhp itself
I am using:
<?php if (isset($image)) echo $this->Html->meta(array('name' => 'og:image', 'content' => $image)); ?>
and it generates
<meta name="og:image" content="https://test.com/img/sharer/startseite.jpg"/>
Sadly Facebook doesn´t like the /> at the end (related here Facebook Open Graph Error - Inferred Property )
I have to use it with a space - but CakePhp generates it that way. Any ideas?