I just figured it out myself. ?? For future reference: the problem is in the funtions.php
add_theme_support( ‘post-thumbnails’ );
add_image_size( ‘gateway-post-image’, 712, 440, true );
add_image_size( ‘gateway-page-image’, 712, 440, true );
as you can see I change both values to the desired size. Now everything works fine.
Thanks again though.