Hi there,
could give this a shot:
1. Create a new Hook Element, select the WP_head hook.
2. Add this content:
<?php
$page_id = get_queried_object_id();
if ( has_post_thumbnail( $page_id ) ) :
$image_array = wp_get_attachment_image_src( get_post_thumbnail_id( $page_id ), 'full' );
$image = $image_array[0];
echo '<style type="text/css">
.main-navigation.slideout-navigation {
background-image: url(' . $image . ');
background-size: cover;
background-repeat: no-repeat;
}
</style>';
endif;
?>
3. Check execute PHP and set your display rules.