functions.php

<?php
$page_meta = new_cmb2_box([
    'id' => 'page_meta',
    'title' => 'Page Options',
    'object_types' => ['page'],
    'context' => 'normal',
    'priority' => 'high',
    'show_names' => true
]);
$page_meta->add_field([
    'id' => 'via_mobile_featured',
    'name' => 'Mobile Featured Image',
    'desc' => "Add a header image for mobile devices only. Won't show without a featured image assigned.",
    'type' => 'file',
    'options' => [
        'url' => false
    ],
]);

header-image.php

<?php
$mobile_thumbnail = get_post_meta($post->ID, 'via_mobile_featured', true);
$thumbnail = get_post_thumbnail_url('full');
?>
<style type="text/css">
<?php
if (!empty($mobile_thumbnail)) {
?>
.page-header {
    background-image: url('<?php echo $mobile_thumbnail; ?>');
}
@media (min-width: 768px) {
    .page-header {
        background-image: url('<?php echo $thumbnail; ?>');
    }
}
<?php
} else {
?>
.page-header {
    background-image: url('<?php echo $thumbnail; ?>');
}
<?php
}
?>
</style>