Found a little Bug in 4images.
It's not very critical, just if you use an non-standard user table fields.
Then the user online won't be showed.
Fix:
Search this line in includes/session.php
if ($row['session_user_id'] != GUEST && (isset($row['user_id']) && $row['user_id'] != GUEST)) {
and replace it with
if ($row['session_user_id'] != GUEST && (isset($row[$user_table_fields['user_id']]) && $row[$user_table_fields['user_id']] != GUEST)) {