function THEMENAME_preprocess_node(&$vars) {
  $lang = $vars['node']->language;
  if ($vars['page'] && $vars['node']->type == 'some_type' && count($vars['node']->field_image[$lang])) {
    drupal_add_css(
      'body { background-image:url(' . file_create_url($vars['node']->field_image[$lang][0]['uri']) . '); }',
       array('type' => 'inline', 'group' => CSS_THEME)
    );
  }
}

Drupal 7.x