Use customfield Image as background in Oxygen

I need help MetaBox in Oxygen:
I will use MB Settings (name: website) to make a custom field for the header image. I have set the field type to image(name: image_header_large).
The function I have make is :

function mb_website_img( $field_id ) {
    $images = rwmb_meta( 'image_header_large', ['object_type' => 'setting'], 'website', array( 'limit' => 1 ) );
    $image = reset( $images );
    return $image['url'];
}

In the PHP Return Value of the BAckground Image I have set mb_website / default value (for now).
No image will appear in the background. I thing the code is not correct.