Is it possible to record user agent information on node creation?
I would probably use a combination of hook_node_insert and watchdog (to keep UA info off of nodes and because it is really a logging exercise).

Example (this is totally untested and off the top of my head):

function MYMODULE_node_insert($node){
  if ($node-&gt;type == 'MY_NODE_TYPE') {
    // Check for the empty value however you want here.
    // e.g. Field API, EntityMetaDataWrapper, or whatever
    if (!isset($node-&gt;field_MY_IMAGE_FIELD[$node-&gt;language][0])) {
      $user_agent = $_SERVER['HTTP_USER_AGENT'];
      $account = $_GLOBALS['user'];
      $message = t('Empty image field created by !uid using !user_agent',
        array('!uid' =&gt; $account-&gt;uid, '!user_agent' =&gt; $user_agent));
      watchdog('MYMODULE', $message, WATCHDOG_NORMAL);
    }
  }
}