The simplest (not necessarily the cleanest) way to do this is:

function mytheme_preprocess_html(&$vars) {
  // Try to get a node object associated with the page.
  $node = menu_get_object();
  if ($node->nid) {
    // We're on a node page
    if (isset($node->field_background_image[$node->language][0]['fid'])) {
      // Load the file object
      $file = file_load($node->field_background_image[$node->language][0]['fid']);
      
      // Get a web accessible URL for the image
      $path = file_create_url($file->uri);

      // Add the background to an inline CSS tag
      drupal_add_css('body { background:url(' . $path . ') left top no-repeat !important; }', 'inline');
    }
  }
}

The simplest (not necessarily the cleanest) way to do this is:

function mytheme_preprocess_page(&$vars) {
  $node = menu_get_object();
  if ($node->nid) {
    // We're on a node page
    if (isset($node->field_background_image[$node->language][0]['fid'])) {
      // Load the file object
      $file = file_load($node->field_background_image[$node->language][0]['fid']);
      
      // Get a web accessible URL for the image
      $path = file_create_url($file->uri);

      // Add the background to an inline CSS tag
      drupal_add_css('body { background:url(' . $path . ') left top no-repeat !important; }', 'inline');
    }
  }
}