Habs mittlerweile selbst rausgefunden. Wen es interessiert:
<?php if (has_post_thumbnail( $post->ID ) ): ?>
<?php $thumb = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'full' ); ?>
<style type="text/css">
.header {
background-image: url('<?php echo $thumb[0]; ?>');
}
</style>
<?php else : ?>
<?php $image = get_field('normal'); ?>
<?php $imagehover = get_field('hover'); ?>
<style type="text/css">
.header {
background-image: url('<?php echo $image['url']; ?>');
}
.header:hover {
background-image: url('<?php echo $imagehover['url']; ?>');
}
</style>
<?php endif; ?>
<header class="header" role="banner" itemscope itemtype="http://schema.org/WPHeader"> ... </div>