<?php
$id = get_the_ID();
if (has_post_thumbnail( $id ) ):
$thumb = get_the_post_thumbnail_url($id, 'full');
elseif (tribe_event_featured_image( $id)):
$thumb = tribe_event_featured_image( $event_id, 'large', false, false );
else:
$thumb = get_template_directory_uri().'/images/background.jpg'; endif;?>
<body <?php body_class(); ?> style="background-image: url('<?php echo $thumb;?>'); background-size: cover;">
?>
Also you don't actually need the `elseif` statement, `tribe_event_featured_image` is a wrapper for the post thumbnail image with some added wrapping html.